Don’t be friends with people who hate others
[The host of] Kamtza and Bar Kamtza were good friends. Suddenly, something happened. [The host] went and embarrassed [Bar Kamtza] in front of all the people at the meal.
“I’ll pay for everything!” [said Bar Kamtza.]
What goes on in a person that he has a friend, called Kamtza, but can hate Bar Kamtza?
The person he hated was the son of his friend! Why was it called ‘Kamtza and Bar (i.e. ‘the son of’) Kamtza? Kamtza was OK, but Bar Kamtza wasn’t OK.
How can you be friends with someone who hates your own son?!
What did Kamtza do? He was a beloved friend [of the host]. But how can you be the friend of someone who hates Bar Kamtza [i.e. your own son]?! [From this we see] that if you someone’s friend, Jerusalem can also be destroyed because of this, too. This is the chiddush (original Torah thought) here.
Kamtza and Bar Kamtza destroyed Jerusalem, because how can you be the beloved friend of someone who hates someone else?!
If you see someone who hates other people, don’t be his friend!
This is the explanation of Kamtza and Bar Kamtza. You only strengthen [his hatred of other people] if you are his friend.
Translated from Rav Eliezer Berland’s recent comments to a gathering in Elad, Iyar 5778.
 In the story of Kamtza and Bar Kamtza (Tractate Gittin), the wrong person was invited to a festive meal. The person throwing the meal meant to invite his friend Kamtza, but the invitation was wrongly given to his enemy, Bar Kamtza, who duly appeared. When the host discovered the mistake, he ejected Bar Kamtza from the proceedings, despite Bar Kamtza offering to pay for the whole meal, if the host would not embarrass him publicly by forcing him to leave.
The rabbis of the time were also at this meal, and didn’t publicly protest the host’s bad treatment of Bar Kamtza. Bar Kamtza later went to the Romans and told them the Jews didn’t want to sacrifice their animals in the temple anymore – which Bar Kamtza proved by making a small nick in the eyelid of the ram the Romans gave him to take to the temple. The Gemara sites the episode of Kamtza and Bar Kamtza as an example of the sort of sinat chinam, or baseless hatred, that caused Jerusalem to be destroyed.