Murders in any state is a crime. It is one of the worst and treacherous crimes to murder someone while one is worshiping or offering prayers.

(وَمَن يَقْتُلْ مُؤْمِنًا مُّتَعَمِّدًا فَجَزَاؤُهُ جَهَنَّمُ خَالِدًا فِيهَا) (النساء:93)

But whoever kills a believer intentionally – his recompense is Hell, {Sura Nisa:93}

(وَلَا تَقْتُلُوا النَّفْسَ الَّتِي حَرَّمَ اللَّهُ إِلَّا بِالْحَقِّ) (الإسراء: 33)

And do not kill the self that Allah has prohibited, except by truthful (right) {Sura Isra:33}

(وَمَا كَانَ لِمُؤْمِنٍ أَن يَقْتُلَ مُؤْمِنًا إِلَّا خَطَأً) (النساء: 92)

Never should a believer kill a believer; but (If it so happens) by mistake, (Compensation is due): {Sura Nisa:92}