When his first shot doesn't immediately kill her, he's already gearing up for a second one. He doubts she'll survive the first one, not with where he hit her - but he doesn't want to leave her to bleed out. Whatever the Capitol thinks of him, Derek has never been purposefully cruel.

But then he hears the sound of someone running, and his attention is split towards the new threat. Another troll, one who only has eyes for the one that Derek just shot, and he ruins Derek's clean line of sight as he skids to a stop at her side.

There's no reason for him to be here anymore. He'd taken out the immediate threat, he should go back to the other soldiers and report that the location of the bomb may be compromised and they need to either up security or move it. He backs away, using the other troll's distraction to make his escape.