I think it is a big sofek who is the real guily party by the story of kamtza bar kamtza specifically.
1)the servant, who started the feud by giving an invitation to the baal simchas enemy
2)The baal simcha of course
3)the rabbonim at the seuda who did not stop the humiliations
It could be all the above, even kamtza. As far as sinas chinom, that was prevelant among the yidden, and this story of Kamtza was both an example of sinas chinom, and a direct result of the churbon (since kamtza snitched on the jews to the roman emperor etc.)