Wow. Just wow.
OP, you did exactly the right thing and were not in the wrong. If you set up in their campsite that's obviously not Ok, but taking the one next to them, especially when there are no others, is not a problem. And even if it were, the other guys were the ones completely out of line with their reaction.
The OP alluded that it was the same campsite as all the other ones were taken. He does state that this group may have tried to take two but then again as long as there is a tent per campsite, there is nothing wrong with it. For all we know, they were friends that went on 2 distinct camping trips but staying at same campgrounds and wanted to hang out a bit one night. We don't know
No, he did not do the right thing initially. He did the right thing after conflict arose. It was not right to set up camp at a site that was already established by someone else however
I mean really, why didn't he share the camp with people that he said were cool with it than risking setting up with others and not seeing if it were ok?
Yes, but there was no other campsites. they were all taken with even more people and none of them had a problem sharing with whoever was there.