The King of Bel is the demon king. I rule over all the demons as their overlord. [It's a necessity for him to become the king. If he didn't, the other Bel demons would take the throne and leave the world in chaos.] I choose to fight God because when I win, humanity won't have to worry about Him any more.

[He gave up his own freedom and normal life to fight for them. A fact that he won't ever forget in Luceti, where he can have some semblance of a normal life.]

That's why I would choose to take down Polaris. Let humans protect themselves.