Negris was surprised as he projected along with Ange's thoughts, "Huh? You've made progress, you didn't kill him immediately but asked for clarity first?"
One must admit that this was progress for Ange. Although his tone was angry, it was clearly an interrogative sentence; he did not act immediately.
This was crucial because Ange's Soul Brand had already been imprinted onto the Little Demon King's consciousness, establishing a Soul Contact, which allowed Ange to project himself. This involuntary Soul Contact meant that as soon as Ange had a thought, the consciousness of the Little Demon King could be wiped out, leaving him without the ability to even retaliate.
Luckily, Ange wasn't impulsive, or else their painstakingly successful ambush on the Ancient God Hunter might have ended in a significant loss.