The point of the crusades was not to drive back the muslim hordes it was the catholic church deciding they wanted to take back the city of jerusalem, which had been in muslim hands for ages. They felt they had the right to the city of Jerusalem and so they decided to take it. War is never justified, and most of the reasons for war are stupidity at its best.
Got to disagree with this, there were a few aims of the Crusades, the popular one (And the one preached to the population) was taking Jerusalem back. However the underlying motives were to drive back islamic aggressors and aid the Byzantine Empire.
Violence in Chrisitianity is an interesting subject, I can understand total pacifism arguments, however... If a friend of yours is about to be hurt badly/killed by another person and you have the power to stop this, (violently) would you let your friend be hurt?
What if it wasn't all physical and instead he was just being robbed every day by people... and you had the power to stop this violently, would you?
What if they were suffering under an oppressive regime with a dictator who did not allow things such as freedom of speech and persecuted & threw in jail anyone who said anything that wasn't his political views? Religious views even? Would you stop this had you the power.
What about the American civil war & slavery?
It's very tricky subject and to be honest I'm not sure what our christian response should be - but I do not think we should be standing idly by and letting people persecute & suppress others at will. Some people (don't know about myself) might even consider it their christian duty to protect others. Evil flourishes when good men do nothing... (yeah I know it's not biblical) and after all, Israel fought to protect itself & went to war in ancient days & participated in mass killings.
- Edit -
Also there is a significant difference between the different crusades and their reasoning for going, I think the discussion here is primarily focussed on the 1st crusade. The others had different reasons for going and heck the 4th one sacked Constantinople.