All perfect praise be to Allah, The Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
The ruling in this regard is based on the scholarly difference regarding the ruling on the Muslim who does not pray and whether he is considered a disbeliever or only a sinner. The scholars who held that such a person is accused of Kufr (disbelief) that takes him out of the fold of Islam also held that he cannot be a martyr. Those who held that he is a sinning Muslim also held that he can be a martyr. For more information about the ruling on the Muslim who abandons the prayer, please refer to fatwa 84235. The second view (that he is only a sinner) is the one that is held by the majority of the scholars, and it is strong and has considerable proof.
The Muslim who is wrongfully killed may be considered a martyr. Abu ‘Umar ibn ‘Abd Al-Barr commented on the statement of ‘Umar ibn Al-Khattaab, "O Allah! I ask you for martyrdom in Your way and death in the city of Your Messenger." He said, "This statement indicates that the Muslim who is wrongfully killed is a martyr whether he was killed on the battlefield or elsewhere, in non-Muslim lands or elsewhere." Please also refer to fatwas 83878 and 88945.
Allah knows best.