Is it forbidden to pray for someone's death?

It is permissible to supplicate against the disbelieving enemies of Allah for their death when they persist in their defiance and there is no hope for their guidance, as was done by the prophets and the companions. Similarly, it is permissible to supplicate against oppressors, even if they are Muslims, although forgiving them is better. Examples of this have been reported in the Sunnah (definition). It is emphasized that the supplication of the oppressed is answered, but a supplication made without right will not be answered.