Remember why he became so powerful? Because his selfishness was sky-high that somehow make his magic capacity and capability huge. But originally, he was not. Actually he was suffers from some kind of sickness. So, he went back to the past to cure that (Or maybe he just make a stroll to the past).
He actually intended to give his past self good health and strong mental fortitude, but because he was bad at mind manipulation, he just made him snap. I know, this is just a paradoxical mess!