Poaching is common here (SC) and I've never heard of a poacher ambushing a witness. Nor do people "normally" hunt/poach with semi-automatic weaspons, nor do people usually poach by riding around shooting out the back of a truck. Poachers here don't want to ruin the hides because they sell them. Poaching animals isn't at all like thrill killing.
My sense is that it might not exactly be a "poaching" scenario. That would imply that the perps had their wits about them, and were consciously "hunting." It's not that hard to imagine a couple of jerks shooting off their guns just for the hell of it, and something went dreadfully wrong. Just a possibility...