The problem between muslims and christians and jews is much older than the Crusades, so I don't think that is it really either.
Satan exists, thus so does evil, and it has throughout history. we are only responsible for ourselves. the muslim terrorists were given free will just like everyone else. Everyone has to choose whom they will serve.
In the early middle ages, Christendom was under siege by wave after wave of barbarian Muslims from the south, Vikings from the north, and Avars and Magyars from the east. In the 11th century, Christendom began to pull out of the dark ages and went on the offensive against Islam: the reconquista of the Iberian peninsula, the Normans wrested Sicily from the Arabs, and finally the First Crusade.
It's ridiculous to blame Muslim anti-Christian sentiment on the Crusades: They were on the offensive against Christendom from the 7th through the end of the 17th century. They went on the offensive again with the taking of American hostages in Iran in 1979. They started the fight and we'll have to finish it.
Deus Le Volt, baby.