It’s true that Christians committed atrocities. In war (and not in war) people often commit horrific brutalities - something God hates. Over the 400 years before Christians reached the gates of Muslim-held Jerusalem, Muslim conquerors had killed tens of thousands of Christians, subjecting them to slavery and worse. They had burned or sacked the holiest churches in Christendom and had pillaged Jerusalem and the surrounding lands. Christians, upon winning the battle in Jerusalem, entered the city and killed approximately 3,000 inhabitants.
We consider this behavior barbaric, evil, and completely inconsistent with the teachings of Jesus. So how could this happen? The simple truth is that claiming the name of Christ does not immediately produce Christ-like character. People who call themselves Christians can sin (even kill) in the name of Christ. Becoming a true Christian means acknowledging that we are sinners in need of the saving forgiveness and corrective lordship offered by Jesus Christ. From there, much growth is needed, and even sincere Christians are still capable of sin.