One classic method is to try to tie it to scripture, but people behave like people regardless of their scriptures, and members of every religion has done plenty of horrible things in the name of their god. The Christian Gospels are about as pacifist as scripture gets and yet many horrible things have been done in Christianities name - often justified because the victims were non-Christian.
There is plenty of religious-sanctioned violence in the Old Testament, including in the Torah, and plenty of violence against non-Jews today - so much that it's a bit bizarre to overlook it.
I mean, we have modern day Islamic societies that are still dragging out homosexuals and stoning them, and still putting women in bags as property.
If that was what I needed to defend with a straight face, I also would be pretty desperate to shift the focus back a thousand or so years.
At what point does reality become more important than your favorite childhood stories?