What is the ruling on praying for someone's death and then they die? And is the one who prayed sinful if they said: "O Allah, do not prolong his life if You have not decreed guidance for him"?

Praying against a specific disobedient Muslim is not permissible, unless he is an oppressive and harmful person, in which case it is permissible to pray for his death. However, praying for Allah to destroy him if guidance is not decreed for him is permissible; in fact, it is a relief for him to avoid accumulating many sins. Prophets like Noah and Moses prayed against their peoples after they became certain that they would not believe. Nevertheless, it is better to pray for guidance. As for the death of another person while attempting to stand with him, it was not due to the one who supplicated, and there is no sin upon him.