I think I'm more inclined to agree with Zodiac. We hardly know anything about whether or not the power scale among the demons is really uniform. Their opponent might be levels weaker than the ones Laxus and Natsu fought. And Laxus almost curb stomped his opponent, where Natsu didn't really have a huge advantage and still managed to win. Don't get me wrong, the demons are likely powerful enough but not as much as they boast to be.

Zeref's line there... meh. Likely it's just a matter of being merciful or ridding the world of evil? And because Natsu is Natsu he won't kill anyone for sure... I'm not really anxious about it, I'll have to admit.