I think what you did was a heroic and brave thing. If they figure out that you were the one who had reported them they might cause trouble for you and get back at you. I think this is what most people would fear when faced with a situation like this. People would rather not get involved due to the fact that it might complicated their lives. Killing animals just for fun should not be tolerated even if the animal that they killed was dangerous. Even animals kill for a better reason. Those people deserve to be apprehended. I hope they learn their lesson and not do it again.