Actually, The Crusades were launched in response to an appeal for help by the (Christian) Byzantine Emperor. He was facing encroachment by the Seljuk Turks into his eastern territories (modern day Turkey). Unable to contain the threat entirely on his own, the emperor appealed for help from Western Europe. The First Crusade was raised in direct response to that call.
However, the crusaders showed little inclination to deal with the Seljuk Turks. Instead, they marched on from Constantinople (modern day Istanbul), intent on carving out empires for themselves in the Near East. Rather than helping their fellow Christians in Constantinople, a later crusade even sacked that city, delivering to the Byzantine Empire a blow from which it never recovered.
Crusader rule in the Near East was even more heavy-handed than that of the Muslim (Arab) rulers they supplanted. This was especially true of their treatment of the local Christian communities, which were quite significant in number. Brutality on the part of The Crusaders was only exceeded by that of the Mongols, who invaded the Near East several centuries later. These Mongol armies themselves contained a number of Christians, mainly from the Nestorian branch of Christianity.
Of course, excesses on the part of "Christian" Crusaders and others in no way excuse acts of terrorism, such as we have just witnessed in Westminster.