Question: If a man rejoices at the killing of a man for a religious reason or for a worldly reason, does he bear any consequence for that?
The answer: Rejoicing at the killing of a man because he is a person of innovation, or a person of sin, or because of his harming the one who rejoices or others, or the like of that – is permissible, and there is no harm in it.
But if the rejoicing at his killing is on account of his faith, his piety, or his knowledge, then that is not permissible; for the believer is not truly a believer until he loves for his brother what he loves for himself, and hates for him what he hates for himself.
Source: Min Thimār al-ʿIlm wa al-Ḥikmah vol.2