Jack hasn't really cottoned on to the fact that Spike's doing this to help save other people rather than just because it's fun/he likes a fight/to save his own ass (delete as applicable). He certainly seems to be enjoying it, given the relish with which he's punching at the raptor and the glee with which he snaps its neck.
Spike's so gleeful that Jack doesn't even realise he's injured too. He flicks a glance back at him when he makes that remark, even as the raptor he felled finally goes limp and he darts in to retrieve his knife.
"Why, you going somewhere?" he asks glibly, diving after the straggler as it belatedly starts to turn and run. In fact, he ends up diving right onto it, clinging to its back and being dragged along as it struggles to get airborne with his extra weight on it, while he tries to get his arm underneath it to stab it somewhere the knife won't just bounce off.