The most rebellious of people against Allah, Exalted and Glorified, is someone who kills another in the sacred precincts of Allah; kills someone other than his killer; or someone kills someone out of a pre-Islamic animosity. [Al-Bukhari]

"إن أعتى الناس على الله ثلاثة: من قَتَلَ فِي حَرَمِ اللهِ، أَوْ قَتَلَ غَيْرَ قَاتِلِهِ، أَوْ قَتَلَ بِذُحُولِ الجاهلية".   (البخاري)