Why do you believe that God allowed so many Jews to be killed in the holocaust?
God did not “allow” any Jews to be killed in the holocaust. By rejecting Him they put themselves outside His protection. Since they are His chosen people this made them a high priority target for the enemy, and he showed no restraint in his effort to destroy them.
A prophecy in Hosea 5:15 shows that after they rejected the Messiah He would go back to His place until they admit their guilt and seek His face.
I’m convinced Satan thinks if he can destroy the Jewish people before they do this, he will have prevented the Messiah’s return and defeated God.