What is the ruling on attending the janazah of ones who are killed and their situation is that they died doing haraam activities?
It is permissible to offer salaatu janaazah on such people. However, it is upon the leaders of the community to stress the gravity and severity of their crime and to warn against it and its likes.
Ash-Shaykh Albany, rahimahullah, said:
والزاني ومدمن الخمر ونحوهم من الفسّاق فإنه يصلى عليهم
“… He who is a person of fornication and addicted to drinking alcohol and the likes of them from the sinners are to be prayed upon [i.e when they die]…”
[Ahkaam al-Janaa’iz, page 83]
The Lajnah ad-Da’imah stated:
A: According to the Madh-hab (School of Jurisprudence) of Ahl-ul-Sunnah wal-Jama`ah (those adhering to the Sunnah and the Muslim community) amongst the Sahabah (Companions of the Prophet, peace be upon him) and the Salaf (righteous predecessors) who followed them, those who commit a major sin such as murder or suicide are not Kafirs (disbelievers) and thus, the Funeral Prayer is to be offered for them. Proof of this is that the Prophet (peace be upon him) ordered Muslims to offer the Funeral Prayer on someone who misappropriated booty in the path of Allah (Exalted be He).
They also stated:
If a Muslim dies persisting in a major sin, such as drinking alcohol, Riba (usury), Zina (sexual intercourse outside marriage), stealing, or similar sins, or if they commit suicide, according to Ahl-ul-Sunnah wal-Jama`ah (adherents to the Sunnah and the Muslim mainstream), they are considered to be a believer due to their Iman (Faith), but their major sin renders them a Fasiq (someone openly and flagrantly violating Islamic law). The matter will be left for Allah to decide, if He so wills, He may forgive them, and if He so wills, He may punish them according to their sins. The body of the deceased should be washed, shrouded, and buried in the Muslim graveyard after offering the Funeral Prayer for them[…]
A Side Note: The shuyookh have also mentioned that it is likewise permissible to offer condolences to their family and relatives because they did not die upon Kufr [disbelief].
AbdulFattaah bin Uthman