Why's Big E like that?

Hey everyone! I've recently getting into warhammer lore and I'm enjoying it a lot. A question popped up and I've been thinking about it a lot but can't find a solid answer.

Why's the Emperor the way he is? I mean, why he has such a huge beef with xenos? Even the pacific ones that are just living in their own world without causing any trouble.

Why he forces his will in such violent ways? He's a conqueror, I get it, but he annihilates human beings from planets that refuse to surrender to the Imperium.

I know that no one in Warhammer is "the good guy" and such, but it was said that Big E is a man that did lots of good for humanity during his days on regular earth, and he's also merciful (somehow?), and generally a "okayish" guy like Vulkan.

I'm just very confused. He's a very ambiguous character and sometimes I don't not if he's doing it all for his own greed or truly it is for the sake of humanity.