As far as Derek's concerned, he can't let this rebel go. One more rebel down means one less rebel to help complete whatever their mission is, and in District One, where the rebels are at a disadvantage - one less of them could mean the difference in them winning or failing.

Everything in his movements is purposeful, fully intent on making the kill.

When the troll hisses at him, baring his fangs, Derek snarls back. His own slightly pointed canines are nothing on the troll's, but the growl he makes is closer to animal than human. Derek slashes at the troll's raised arm again, hoping to injure it enough that it makes wielding that weapon he's got more difficult.