Not poachers...

Based on his posessions which he abandoned along the way, they were led to believe the apes had killed him and were jettisoning his stuff as they lost interest. They tracked down the group and set about destroying what they thought were a band of killer apes.

They were poachers. They were making no attempt to call out to see if any man was still alive in the forest. What is deceptive is that they wore some kind of uniform - for park rangers or something -- but their behavior indicated they were corrupt and out to get the gorillas. Basically it was to make more money because they probably didn't make much money in their regular jobs. They came in force with a lot of gun power so they could kill all the adults (who would try to defend the gorilla babies). Then they would sell the babies for a very big price. They probably would also take adult animal parts like the feet to sell, too. Did you notice the bloody knife? When Ethan used the knife to cut foliage, it was clean. These men found and used the knife to cut up gorillas. They were following the trail of an American scientist that they no doubt found out was studying the gorillas. They didn't care if the man was alive or dead. They weren't going through the forest searching for him to save him at all.