Well, that's an answer. Unfortunately for Sebastian, his aim isn't completely true. He misses the gun, and hits Ankh's arm instead. There's a spatter of blood and a yelp of pain, and for a moment, it's so intense that he can't keep his focus on Sebastian. He certainly can't keep the gun aimed at him. But at the same time, much as he didn't initially have any desire to hurt Sebastian, that wound is more than enough reason and he was about to shoot him anyway.

He brings the gun back up through sheer force of will, using his other arm to brace his grip, aims, and fires.