Dude, he’s a meta-human in his own right, has kick-ass powers, and decided to fucking name himself “anti that fucking guy”. I don’t think you’re going to find much deeper motivation than just… hatred. Unadulterated loathing.

Anyways, I see the most common reverse flash lore is that he’s from the future, giant flash fan boy, then finds out he’s the guy who goes around fucking up the flash’s life instead of becoming his successor, and simultaneously finding out he’ll never achieve his lifelong dream and that the flash is the culprit (since he’s the flash, who inspired him in the first place) caused him to snap and hate everything about the flash.

He’s literally a giant hater because he was always going to be a giant hater. There’s nothing purer than that.