Is a prayer against someone answered due to anger, and does sin result from that?

It is permissible to supplicate for the death of an oppressor if his injustice is clear and warrants such a supplication. However, if the injustice is minor, it is not permissible to supplicate for something greater than the injustice itself; otherwise, the supplicant would become an aggressor and sinful. Similarly, if there was no injustice to begin with, but merely an ordinary dispute, then the supplication in such a case is considered injustice and aggression.

If the supplication is made in a state of distress and annoyance without genuine intent, it is not expected that harm will befall the one being supplicated against, because Allah, in His mercy, does not answer the supplications of His servants when they are in extreme distress and anger, as confirmed by His saying, the Almighty: "And if Allah were to hasten for the people the evil [they invoke] as they hasten the good, their term would have been concluded for them."