The Crusades - Christianity is to Blame
Christianity can and should indeed be criticized because of the crusades.
(Luke 19:27) "But those enemies of mine who did not want me to be king over them - bring them here and kill them in front of me"
Now that has a clear message. It condones violence. That is in the book of what is supposed to be God's influence. When a brutal event is then said to be as a result of the guidance of such a message, that message must be criticized. Not necessarily as the cause of the event, but as a valid excuse for that event.
If you said that I was the holder of all power and knowledge, and referenced that information to something considered valid by others (like the bible) and said "I did that because Philanthropy2012 told me to do it" and whatever action it was, it was clearly immoral, then I should be criticized for telling you to do so, even if your actual reasoning to do it was different from what you blamed it on.
Another example is that of a parent who tells their child that it is okay, and that in fact she should steal dad's sweets. The child then goes ahead and steals dad cheats and is caught by dad in doing so. When asked, the child says that she only did it because her mother told her to do it. The father then would tell the mother off for condoning such behaviour, fully in the knowledge that the child only did it to attain sweets.
The same it is then for christianity and the crusades. It was done in the name of God, who condoned the behaviour, perhaps albeit for greed or personal beliefs, but both parties involved need to be criticized for their actions.
Argument from Interpretation
Please don't argue about how the quotes clearly endorsing violence should and could be interpreted as something else and give an extremely elaborate explanation as to why it's not actually doing so.
The bible was not written by idiots. If you believe that it was, then why are you following it's guidance? If they had wanted to convey a message, they would have. They didn't need to write the book of God in code for people to decipher.
Argument From Contradiction
The fact that Jesus seems to teach the opposite in other parts of the bible has no relevance either. What you then have is a completely contradictory book.
A book written with the distinct purpose of allowing any event to occur under it's name at any time. Once again, such a book should be criticized for ambiguity. Any religion founded upon such ambiguity is liable to such criticism too. The fact that it was there as a very valid excuse for committing crime is a fault in itself.
The words clearly state "kill them in front of me."
Argument From Translation
Now, you might argue that the quotes condoning violence are mistranslations, but from what?
And if such fundamental mistakes in translation were made, that would convey the exact opposite meaning, then how can you trust anything else written in the bible? If the meaning can be so easily skewed, any belief that is based on the bible can be questioned and changed based on the assumption that it was translated incorrectly?
How could such a drastic mistranslation be made? And what was Jesus' actual statement?
"Dance with them in front of me" ? Please.
Moreover, there have been many translations of the bible into english, and each convey the same horrific meaning of Luke 19:27
The different translations are listed in the link below.