And that the trespasser had been told to leave and he was staying to make sure he went. Don't forget that part. They had, at that point, every reason to believe that when they got there they would find Wilkers by himself, the trespasser having already left.
when they left the cabin they are now a posse on a mission of removing a subject from said property,
No, they are not. You continue to imagine things that are not there. There is nothing in the statement that says that. It says that they went to join Wilkers but it does not give a reason, for all you know they were going to see if any damage had been done to the tree stand a far more likely possibility then your wild imaginings.