Be prompt in intervening when someone does something wrong is what's implied by the following:
-What is “senseless hatred”?
Rashi on Shabbat 32b: It is hatred of people who have not committed any action justifying the hatred.
-When does the hatred make sense?
Pesachim 113b: You may bear ill will towards someone if you witnessed him engaging in a forbidden act.
Bava Metzia 32b: But you must assist him before assisting those you love [so that ill will does not turn to hatred.]
Sanhedrin 27b: A hater [sone] is someone who does not speak to his fellow for 3 days because of enmity.
When the Talmud says: "The Temple was destroyed because of senseless hatred", the common interpretation is that Jewish factions were fighting among themselves and made it easier for the Romans to win the war. But elsewhere, the Talmud says that the senseless hatred was a lot more focused: The war happened because of a strange backwater chain of events that wouldn't even have made the newspapers. [See Gittin 55b-56a and Lamentations Rabbah 4:3]
If you read the story, it's clear the host did not properly engage Bar Kamtza (who accepted the invitation because he thought it was an olive branch). There is the senseless hatred. Had he engaged him and cleared the misunderstanding as Bava Metzia 32b (above) says, that would have been the only proper way to proceed.