And as I pointed out, Christians embrace a lot more than just the New Testament. You won’t find the words of Jesus in the Old Testament; and Christians behave properly by not using it as justification for their actions. But many still do.
From that perspective, they may not be following Jesus’ teachings. From the Jewish point of view, as well, the Mosaic covenant does not apply to Christians, it is only for Jews. The Christians belong to the Noachide covenant. But Christians still abide by the Ten Commandments, even though Jesus never mentioned them as Christian doctrine.
And this is the dark side of Christians: cherry picking the doctrines of the Hebrews and choosing those parts that they like, but are not Christian.
There’s a difference between a dark side to some who profess Christianity and the teachings of Christ Himself. Those who practice that dark side are NOT followers of Christ, whatever label they use to try to justify or excuse themselves.
Jesus Himself said He wouldn’t recognize them.
Islam IS a dark side. I haven’t seen much in the way of teaching from the Koran, and there’s been enough posted on FR, that shows any sweetness and light.