Shes asking who he thinks he is by coming back and trying to get back with her. She doesn't want to go back with him and shes angry hes even trying to get with her again. like, "Who do you think you are?!?!"
He literally went around tearing apart love. Shes basically saying in this part that he is a homewrecker and causes many people pain. He causes break-ups and heartaches all over, and hes done it to her. He tore apart the love they shared.
