I simply disagree that most of them were dead set on him cheating. One guy is dead set on him cheating, Kibaou. He's racist against beta testers xD. What a lot of the others seem to be, is easily swayed. This means that a solid argument that's simple to get could sway them in the proper direction. We saw this happen already when Egil mentioned the booklet of data compiled by the beta testers. That put Kibaou right in his place after he tried to spread his paranoia the first time and it united everyone else.
After the fight, Kibaou came with his accusations, which are essentially the same, just this time, he could pinpoint a beta tester. Egil and Kirito on the other hand, had much more ammunition to argue and defend. First of all, the information concerning the boss and his tactics were accurate save for the weapon change, which Kirito noticed and warned about as the fight unfolded. That's why yelling it out and people being able confirm that he did was important. It shows that he was as forthcoming to the group as he could be. Second, Kirito realized that Diabel was a beta tester, and its clear that Diabel did do something noble in being relatively selfless in the use of his knowledge, even if he attempted to get the bonus phat lewt for himself. Third, he risked a lot himself and took much damage that fight, so it's not like he didn't contribute and risk much. Save for Diabel, he was the most important factor in them leaving that fight alive. All this would show that at least some of the beta testers, the ones present at that fight in particular, weren't terrible people.
Basically he was in a very good position to go up against Kibaou and sway the group. That position was solidified with Egil and whoever else had the wherewithal to listen and think. They would then argue on his behalf as well. All this actually gives Kirito the upper hand. Furthermore, if he decided to go on the offensive, he was in a very good position to put Kibaou in his place once and for all.
"This is the second time you've tried to spread your fear and bigotry, and the second time your cowardice exposed. I think I speak for all of us when I say we've had enough of that."
Again, this is because I did not see anything in the episode indicating that people other than Kibaou were dead set against Kirito and beta testers. They did not seem to be consumed by a mob mentality at that point. Even so though, lets assume Kirito's attempt to sway the group did fail. He *still* could've taken the path he ended up chosing after trying defend himself against the accusations.
As far as the risk he's put himself in, I just don't think it's a good idea to make the world your enemy. News of the events of that fight and its aftermath is going to spread like wildfire. He'll have enemies from the group he was in, and he'll have many more enemies he hasn't even had a chance to meet yet.