[Is One Washed If He Is Killed, or Kills Himself, by Mistake?]

Answer: The school’s view is that one whose death was by mistake is washed—whether he himself caused it or another caused it.
The proof is that the original rule is the obligation to wash the deceased Muslim, except where a specific proof removes him from the generality (as with the one killed in battle). No proof has come for those mentioned in the question; thus one must remain upon the original rule until a contrary evidence comes. And Allah knows best, and praise be to Allah, Lord of the worlds. May Allah bless Muhammad and his family and grant them peace.
Source : Min Thimār al-ʿIlm wa al-Ḥikmah vol.1