I have two "big bads". Technically three if you count the moron who didn't make it. (Sorry Azazel, you failed and almost got killed.) Four, if you count the guy I still don't know what's up to.
And this is just the Chasing Series. In the Eden one, well, they're just fighting each other for Rank.
1. Nanyael - He just wants to get his children back. The fact that they're dead and in Hell? Well, that doesn't stop him from trying.
2. Azazel - He just wants revenge. If it means building an airship and flying it to Heaven to start another war with a bunch of mechanical winged monsters, so be it. Might be why he failed. It was a stupid reason. XD
3. Thelial - He wants to make the world a cleaner place. But it means killing and\or sending to Hell anyone who doesn't accept the brainwashing circlet treatment he offers to them. Oh, btw, he's the new God, because he tricked God into a bet to make Him go away for a while.
As for the fourth - Adouel - He's a demon, who's up to no good, and I really don't know what he's doing, but he's gathering a bunch of demon-nephilim crossbred children to do it.
And I realize there's a lot of el\al's, but yeah, it's an angel\demon world so meh.
Those are all very shallow reasons on the surface, of course, but when you dig deeper and build a lot of story around it, it's much better. I think that's what counts. Not so much the surface reason behind it, but the why behind it.
I mean, why write a book? Because it's fun. Well, that's no reason. But when you actually put it into an emotion many worded response and start doing it, there's no way to describe it. Because it's the most gut-wrenching, agonizingly painful, yet glorious thrillride you will ever send yourself on.
Or something. O_O