Is it permissible to kill a Christian or an infidel?

It is not permissible to kill a disbeliever if there is a covenant, a pledge of safety, or a dhimma (protected status) between him and the Muslims. It is also not permissible to transgress against his property or honor, whether he is a Christian, a Jew, or other. Whoever kills a covenanted person unjustly will be deprived of the scent of Paradise.