Please, smart answers only, thank you.

My work is on the Crusades. I had to take a side about it, and I decided to be against it, meaning I have to find arguments about how bad the methods used were.

I have started, but I'm actually out of ideas.

So, if you can post here a few lines about why crusades should be considered as barbarians invasions, it would be great.
+Full REP to everyone who give me serious infos.