reincarnated as a sealed evil

when a boy dies unfairly to a bunch of bullies his strong negative emotions catch the eyes of a bored god, a god who chooses to reincarnate this boy as a evil sealed inside a young girl, with newly gained absolute power over darkness and a stone cold heart, will this boy give in to the shadow and the plots around him, or will he overpower it and do his best to live a life he chooses?