[Washing One Killed by Mistake]

Answer—and Allah is the One who grants success: The scholars of the school have said that whoever is killed in such a way is to be washed. The proof is that the obligation to wash the deceased Muslim is established in general; one may not depart from this general ruling without evidence.
The evidence concerns only those killed in battle—that they are not washed, as in what is related about the martyrs of Uḥud and other battlefields of the Prophet (May Allah bless him and his family and grant them peace), those whom the disbelievers killed.
And likewise what is related about the martyrs of al-Jamal and Ṣiffīn, whom the transgressing party killed. To this may be attached one killed unjustly or killed while defending his property or another, due to the shared effective cause indicated in what is narrated from the Prophet (May Allah bless him and his family and grant them peace): “Wrap them in their bloods, for they will be raised on the Day of Resurrection with their jugular veins gushing blood …,” and in it: “They will say: ‘Our Lord, ask them why they killed us,’” or as he said.
Accordingly, one killed by mistake or by a stray shot is washed—maintaining the original rule that the Muslim is to be washed.
Source : Min Thimār al-ʿIlm wa al-Ḥikmah vol.1