Why?

Crusades. God also says to kill family members if they're not christians.

That might be Islam, but definitely not Christianity

Old Testament.

The Crusades were a series of defensive wars being waged against an imperialistic Islamic empire that inevitably made its way to the gates of Vienna before definitively being pushed back, spreading sexual slavery, genocide, and oppression everywhere they went like for my Orthodox people who suffered 400 years of Ottoman occupation. Don't get me wrong. You're right that Christianity hasn't been a peaceful religion but your reasons for that conclusion are wrong. It's amazing how much Enlightenment era historical revisionisms like blaming the Crusades on Christendom have stuck around in the secular world today. And these are the same people who foolishly say that Islam is just misunderstood. Again, Christianity hasn't been historically peaceful, but Islam is still probably the most violent religion to ever arise. Read about the Islamic conquest of the Indian subcontinent. Btw, Orthodox Christians suffered on both sides of the Crusades. Remember the Sack of Constantinople in AD 1204 and establishment of various Uniate Churches to undermine the native ones. As for the killing family members who aren't Christian, that's clearly a misunderstanding. I know for a fact because there was no Christianity yet at the time of the Old Testament where everyone was Jewish. And as for that, Christians only interpret the OT in light of the new. Christ is the standard by which we read it. The Church Fathers clarified this centuries ago.