They were ALL at fault, including the missing Kamtza who was supposed to be invited. As soon as he realized his son was invited to HIS friend’s home, where his son was NOT a friend, he should have realized that a) a mistake was made or b) maybe the friends was extending an olive branch to the son. In either case, he should have stepped in to ensure that nothing untoward would happen to cause a blowup. Just my opinion.
Bar Kamtza was guilty of being a malshin, the host was guilty of being incredibly ungracious and publicly humiliating a fellow Jew (who was already begging to pay for the entire party, if only he could stay and not be embarrassed), the servant was guilty of…carelessness, and the rabbonim and choshuveh people who witnessed the entire preceeedings and did not take the host aside and give him mussar or calm him down, were the guiltiest of all, ebcause they are supposed to be the role models and the leaders of Klal Yisroel. A host who loses his temper, might do so because his emnotions take over and he does not think clearly (that does not make his actions right, but it explains his response). But a Rov who sees an avla committed, or better yet, ANY Jew who sees someone being treated this way, has an obligation to step in and at least TRY to turn things around and possibly even to make sholom between the two men.