"We'll be back!" he snarled, revealing that it was actually Ari's voice. This suggests a threatening or ominous situation, with Ari possibly trying to intimidate or warn someone.
Fang comments that killing people isn't as easy as it used to be, implying a change in circumstances or perhaps a commentary on their current situation. The scene is from the book School's Out—Forever by James Patterson, hinting at a tense or dangerous moment involving these characters.