Committing Suicide is An Abhorrent Enormous Sin and a Path to Hellfire
The life of the individual in this world is, in principal, needed for successfully achieving the Hereafter as a winner; a matter accomplished through properly knowing the Creator and worshiping Him as He ordered and as per the teachings His Prophets conveyed from Him.
Maintaining the individual's life constitutes the uninterrupted duration of this life with which knowing the Creator can be achieved. Intentionally and purposely interrupting the individual's life negates the sole purpose of the sent Prophets and Messengers specifically being to teach the people the correct knowledge about Allah and how to worship Him--ultimately winning the Hereafter. This is why killing oneself or unrightfully killing another is an enormous sin.
Next in enormity to the major sin of ending ones own life and unrightfully killing another is adultery and sodomy. Adultery is an enormous sin, but is a sin less enormous than unrightfully killing a soul, because it does not disrupt the continuity of life. Rather it obscures the maintenance of the sound lineage and disrupts the laws of inheritance and the other matters, without which the sound life cannot be maintained. Sodomy is also an enormous sin, but is a sin less enormous than adultery, because if people at large resort to homosexuality in satisfying their sexual desires that would stop the birth of new life and thus disrupts the aforementioned main purpose. Sodomy is less enormous than adultery because it is desired by males among themselves whereas adultery is desired by both males and females rendering the occurrence of adultery much more than sodomy--hence the disruption of the main purpose is greater.
Because of this underlying wisdom Islam, as well as all other religions, judges these matters as abhorrent enormities that lead to destruction and severe punishment in the Hereafter.
The prohibition of killing oneself and unrightfully killing another soul is evident in the Qur'an, the sayings of the Prophet, sallallahu ^alayhi wa sallam, and the unanimous agreement of the reputable scholars and the nation. Only an arrogant and ignorant person would speak otherwise.
The Qur'an and the Hadith explicitly show that the one who commits suicide or kills another person unlawfully commits an enormous sin second only to the most enormous sin of all which is associating partners with Allah. The one who deems such killing lawful and dies with that conviction shall dwell in Hellfire forever. The believer who deems it unlawful yet still does it shall be tortured severely for it in the Hereafter. Both judgments are illucidated in the Qur'anic Verses and the Hadiths (sayings of the Prophet sallallahu ^alayhi wa sallam) shown herebelow.
The scholars of Islam who interpreted the verses of the Qur'an and the sayings of the Prophet on this subject such as Abu Hayyan, an-Nasafi, al-Ghazali, al-Bayhaqiyy, Qatadah, al-Hasan al-Bisri and others, highlighted the seriousness of this sin and the strictness of the resulting punishment. The Scholars established that the one who is in his right mind does know that it is prohibited to commit suicide or to unlawfully kill another person. All the religions agree unanimously that it is forbidden. How killing a person unlawfully could not be forbidden when Islam prohibits the unlawful killing of an animal?
Some try to find justification in the hadith of al-Bayhaqiyy pertaining to the incident of al-Husn (fortress) when a man in the battle jumped down from the wall of the fortress and died, and yet he was judged as not accountable. The scholars explained that his intent was not to commit suicide, rather he wanted to land inside the fortress to open the gates for the army to enter and that it is clearly understood that his assessment was that he could do it without killing himself.
Allah said in Surah An-Nisa', Verse 29: It means: O you who believe! Do not consume your property among yourselves unlawfully, except that it is trading by your mutual consent; and do not kill yourselves; surely Allah is Merciful to you.
Allah said in Surah Al-Baqarah, Verse 84: It means: And when We made a covenant with you: You shall not shed your blood and you shall not turn your people out of your cities; then you gave a promise while you witnessed.
Allah said in Surah Al-Baqarah, Verse 85: It means: Yet it is you who slay your people and turn a party from among you out of their homes, backing each other up against them unlawfully and exceeding the limits; and if they should come to you as captives you would ransom them-- while their very turning out was unlawful for you. Do you then believe in a part of the Book and disbelieve in the other? What then is the reward of such among you as do this but disgrace in the life of this world, and on the Day of Resurrection they shall be sent back to the most grievous chastisement, and Allah is not at all heedless of what you do.
Allah said in Surah Al-Ma'idah, Verse 32: It means: On that account: We ordained for the Children of Israel that if anyone slew a person--unless it be for murder or for spreading mischief in the land--it would be as if he slew the whole people: and if anyone saved a life, it would be as if he saved the life of the whole people. Then although there came to them Our Messengers with clear signs, yet, even after that, many of them continued to commit excesses in the land.
Qatadah explained that this verse reflects the enormity of such killing. Al-Hasan al-Bisri explained killing such a soul carries a burden similar to the sin of killing all the people.
Allah said in Surah Al-An^am, Verse 151: It means: Say: Come I will recite what your Lord has forbidden to you-- that you do not associate anything with Him and show kindness to your parents, and do not slay your children for (fear of) poverty-- We provide for you and for them-- and do not draw nigh to enormities, those of them which are apparent and those which are concealed, and do not kill the soul which Allah has forbidden except for the requirements of justice; this He has enjoined you with that you may understand.
Allah said in Surah Al-Furqan, Verse 68: It means: And they who do not make a supplication to another god with Allah, and do not slay the soul which Allah has forbidden except in the requirements of justice, and (who) do not commit adultery; and he who does this shall find a requital of sin.
From the route of Ibn Mas^ud, may Allah accepts his deeds, who said: The Messenger of Allah, sallallahu ^alayhi wa sallam, said: The first among the matters that would be settled among the people on the Day of Judgment is the matter related to blood shedding.
From the route of Ibn ^Umar, may Allah accepts both of their deeds, who said: The Messenger of Allah, sallallahu ^alayhi wa sallam, said: The believer will not cease to be in a broad range of forgiveness in his religion unless he unrightfully slays a prohibited blood. And Ibn ^Umar said: Among the problematic matters that one commits and has no way out of it is slaying a prohibited blood unrightfully. Narrated by al-Bukhari.
Allah Decreed that it is an easier matter for the world to cease to exist than killing a believer unrightfully. Narrated by Ibn Majah.
From the route Abu ^Abdillah al-Hafidh and Abu Zakariyya bin Abi Ishaq al-Muzakki from Thabit bin ad-Dahhak from the Prophet, sallallahu ^alayhi wa sallam, who said: And the one who kills himself with something shall be tortured with it on the Day of Judgment. Al-Bukhari narrated it in the book of as-Sahih from the route of Musa bin Isma^il and Muslim related it from another venue from the route of Ayyub.
From the route of Abu Hurayrah who said: The Messenger of Allah, sallallahu ^alayhi wa sallam, said: The one who (legitimizes) killing himself with a piece of iron shall be holding it in his hand stabbing himself in the abdomen while he is in Hellfire where he will dwell forever. The one who (legitimizes) killing himself with poison shall be holding his poison in his hand in Hellfire sipping it in Hellfire where he will dwell forever. The one who (legitimizes) killing himself by jumping from a mountain shall be jumping in Hellfire where he will dwell forever.
Al-Bukhari and Muslim related from the Prophet, sallallahu ^alayhi wa sallam, the following story :
One man from the previous nations encountered a swelling in his hand and out of fear he took a knife and cut his hand and he bled to death, so Allah, the Exalted, said: My slave initiated wasting his life so I made Paradise forbidden for Him.
From the route of ^Abdillah bin Mas^ud who said: The Messenger of Allah, sallallahu ^alayhi wa sallam, said: There is no soul that unjustly slays another soul except the first son of Adam will bear his share of the sin because he was the first to innovate the unjust killing.
From the route of Jabir who said that when At-Tufayl bin ^Amr ad-Dawsiyy immigrated with the Messenger of Allah, sallallahu ^alayhi wa sallam, also a man immigrated with him. When they reached Madinah the man got sick and took a blade and cut off his knuckles and bled to death. At-Tufayl saw him in a dream in a pleasing situation but his hand was covered, so he told him; How come I see you covering your hand. He said: I was told we shall not correct for you that which you have spoiled. So he narrated his dream to the Messenger of Allah, sallallahu ^alayhi wa sallam, and the Messenger of Allah said: O Allah forgive his hands also. Narrated by Muslim in as-Sahih from the route of Ishaq bin Ibrahim from Sulayman bin Harb.
From the route of Anas from the Prophet, sallallahu ^alayhi wa sallam, who said: The most enormous sins are associating partners with Allah, killing a soul (unlawfully), unkindness to your parents, the fabricated saying or he said the fabricated testimony. Narrated by al-Bukhari in as-Sahih from ^Amr bin Marzuk.
Ibn Hibban narrated in his Sahih from the Prophet, sallallahu ^alayhi wa sallam, that the one who unlawfully kills a soul under protection shall not find the odor of Paradise, and its odor can be found from a walking distance of one hundred years.
Allah knows best.