I wished death for someone and it really happened. Now I’m feeling guilty and want to repent. What should I do?

Only Allah Ta’ala decides when a person will die or not. As long as you are not a direct cause of his death, you are not guilty. But your dua/curse might have been accepted by Allah. You can make up for this by giving sadaqah on his behalf, or read Qur’aan and send its isaal-e-sawaab to him. From now on, wish good for everyone and forgive and forget.