Why was there so much antisemitism in Europe?

I simply cannot wrap my mind around it. We all know that it's really the Romans who killed Jesus, but the Italians do not get blamed for it. It is often said that it is because they were wealthy in Germany and Austria, but that can't be it either, because the Jews in Poland and Romania were not wealthy. It seems to make no sense to say that it is because they were a different race, because that was hardly true, if at all. And, it hardly seems to make sense to say that it is because they didn't believe in Jesus, because Christianity is supposed to oppose killing, so a religious justification for the holocaust cannot be there. Is it really because people in those countries thought that ALL of the jews were communists? That seems stupid, but I guess there can really be no intelligent answer to this.