Why is he the bad guy again?

>>93922456
The Altean Empire had better propaganda.

>>93922456
Probably the whole Omnicide and Slavery thing

>>93922456
Regardless of his initial motivations, he now destroys entire planets to harvest their life force to make himself stronger in a very DBZ esc manner.

He has also started enslaving races and not even giving them the opportunity to join as a full member of his empire. Rather than finding out more about the humans and offering them the chance to join or serve, they instead threw the humans into a gladiatorial arena and experimented on the survivor.