The Disturbance.

"If I want something, I should go and take it." He said with a determined gaze as he dug into the ground. "Everything is for the taking for those who are capable."

Then he said with a chuckle. "Besides, the demon god owes me."

There is no need for him to come up with a justification for his upcoming actions. For him, there is no good or evil. There is only want and the power to acquire what is wanted. 

If he wants something, and he has the power, then he will get it. The fact that he will come into conflict with the demon god as a result is purely coincidental. He is not motivated by revenge. 

Sure, the demon god tried to enslave him. But that's in the past. Legion is only concerned about benefits. He will still do what he is about to do even if the demon god hasn't tried to enslave him. And if there were no benefits to be had, then he might let bygones be bygones.