PLAYER KILLS PLAYER CONDITION

so, i'm new to mcreator and minecraft java modding in general (i did some bedrock mods on a mobile app before but nothing too big) and after finishing my first mod, i'm trying to make a lifesteal mod, and i want to make when player A kills player B player A will gain an item that when eaten they gain a heart, and player B loses 1 heart, i already made an item, but i think there is no way to make the procedures necessary to make 1. the item be given, 2. player B lose a heart, please tell me if there's a way to do so, and if there is please tell me how??