Will someone explain Zeref to me. The whole weight of human souls concept makes no sense to me at all. The more he values life, the more he wants to destroy it? Or is it when he doesn't value human life at all, that's when he starts going crazy? Also this whole thing about consuming souls and stuff.

Zeref is someone who always takes life, whether he wants to or not. And when he starts out, he wants to kill. So he uses his power to kill. But then he learns the value of life and doesn't want to kill anymore. But his magic won't let him not kill, so it keeps killing stuff for him, and he isn't able to control it because he doesn't want to use it. In order to control it, he has to literally stop caring about life. But when he stops caring about life, he starts wanting to kill again. So he makes himself remember how to care for life, thus reverting him to the state where he kills without wanting to.