Simple but amusing genre work from Miller. First Kill is one of those film that doesn't even try to do anything out of the ordinary. From the casting to the setting and plot, you've probably seen it all before. First Kill is genre filler in its purest form, the positive here is that Miller did a pretty decent job.
A dad takes out his son to spend some time in the wilderness. He's going to learn his boy how to hunt, but after a short hike they run into a scuffle between two guys. One of them shoots the other in cold blood. Though they try to keep out of it, the kid betrays their hiding place, which marks the start of a complicated stand-off.
The cast does a decent job (with a somewhat surprising role for Willis), the setting is appealing, and the camera work is nice enough. Miller simply goes through the motions and doesn't do much beyond keeping the plot on the rails, but if you're looking for a decent action/thriller flick, this isn't such a bad choice.