Sinat Hinam has been a problem facing Jews since the time of the Roman Empire. According to tradition, the 2nd Temple was destroyed due to this causeless hatred, specifically the Kamtza-Bar Kamtza issure.
As the story goes, there once was a man who had a friend named Kamtza and an enemy names Bar Kamtza. Now, this man was having a huge party. Anyone who was anyone was going to be at this party: famous rabbis, the Kohanim...everybody.
Naturally, the man wanted to invite his good friend Kamtza. However, his servant made a mistake and invited Bar Kamtza instead.
When Bar Kamtza received his invitation, he was very grateful to think that the host had finally made amends. But when Bar Kamtza showed up at the party, the host took one look and told his servant to immediately eject Bar Kamtza from the premises.
When asked to leave, Bar Kamtza said: "I understand the mistake. But it's embarrassing for me to leave the party. I'll gladly pay the cost of my meal if you'll allow me to stay."
The host would hear nothing of this, and reiterated his demand to have Bar Kamtza removed.