The persecution and execution of Jesus Christ. That is the deep down hatred. Only my opinion.
But... Jesus WAS a Jew, as was all His followers at the time of His crucifixion.
As He said, He came to fulfil the law, not replace it. It was God’s plan all along. The Romans, the Sanhedrin and the Pharisees were all tools in His hands.
He even told Pilate he had no power over Him, save God allowed what was to be.
I don’t see Jews as Christ killers. He made Himself a sacrifice for ALL of us, Jew, Gentile, man, woman.