This isn't entirely true. That may have been how the Crusades first began but that is far from how they ended. So-called Christian armies from Rome sacked Constantinople, committing all the atrocities that go along with that against other Christians, leaving little left which led to the eventual capture of that city by Muslim forces.
Was there sin in the Crusades? Without a doubt.
I've heard this argument before, and it's yet another exaggeration by the left in an attempt to whitewash Islamic aggression. Constantinople stood for 250 years after being sacked by crusaders, the argument that three days of looting contributed to a cities fall two and a half centuries later is absolutely ridiculous. That's like saying if the United States was conquered tomorrow it was because the British burned Washington in 1814. No, it wasn't invading muslims that did it; it was the evil christian crusaders.
And you're taking my statement out of context, I wasn't saying crusaders never sinned; I was stating that the crusades themselves were just. The myth of the evil crusaders invading peaceful muslim lands is one of the biggest lies in history.