I think I read somewhere that in order for the Romans to abnegate their role in killing Jesus, as they transitioned to Christianity, they scapegoated the Jews. Although, it was the people who demanded to crucify Jesus. But of course, anti-semites focused on the ethnicity, instead just ascribing the mob justice to just-- on people.

My guess is that because Jews reject the divinity of Jesus, the Christians found it fair to oppress the Jews. It is not different though to the crusading missions of later Christians in the medieval Europe. The non-Christians refuse to recognise Jesus Christ as both the son of God and god himself, the either they convert, or else be killed or ostracised.