Yeah I did hear a lot of things saying that he was evil but how do we know that?

a mater of debate I'm afraid however read spoiler

We have to kill him.. so "bad".

From what I can gather from the promo video, he rejected the domination of the dragons in preference for a course that brought power to himself. Thus, his motivations are not virtuous, but simply self-serving.
Also - it looks like he has some sort of cult-ish followers. That's a worrisome sign.

I'd say it's likely that he's a bad 'un. In fact, he sounds like the kind of guy the Greybeards were warning you against becoming.

Yeah, honestly it would be the worst DLC ever if they made him good... Talk about anticlimactic

Perhaps he might be something of a tragic hero?

Dagoth Ur much? Not that it would be a bad thing to have another misguided hero/villain.

lets be honest my heroic vampire lord will slay this evil doer so none can stop my rien, all hail lord draco, leader of the dragons/vampires!