Bar Kamtza was the enemy of Kamtza. he was thrown out of a party being thrown and was publicly embarrassed. he had a lot of anger and took it out on all of he Jews by getting the Romans to not like them. this consequently ended up in the Beit Hamikdash being destroyed.
The Emperor sent the Jews an offering. On the way Bar Kamtza put a blemish on it. The Jews didn't offer it up and the emperor knew they were going to rebel.