TREVOR "borrowed" Skewer (aka the Death Scythe) from Princess Neoma without getting Her Imperial Highness' full consent.
Hence, he was determined to make the most of it.
[I'm sure Princess Neoma will kill me later, so why not use Skewer to my heart's content?]
Moreover, he had something that he wanted to try.
"You're being too arrogant, Trevor Kesser," the Ancient Devil said, laughing while shaking his head bitterly. "Just because I let you have my power as the Devil doesn't mean we're on the same level. I'm being lenient to you because I don't want to accidentally hurt Princess Neoma if I unleashed–"
"Oh, shut up," Trevor said, cutting off the Ancient Devil while swinging the Death Scythe. Unfortunately for the Ancient Devil, he wasn't the type to listen to an enemy's nonsensical monologue in the middle of a serious fight. "Let me try my new attack on you!"
The new attack that he mentioned would be the Hellfire, of course.