While your average Crusader was, more often than not, a bloodthirsty and, especially, landhungry thug motivated by a varying mix of religious fanaticism and self-interest peppered by general bloodlust, they were NOT hell-bent to the destruction of the Islamic faith and civilization* as such. Actually, Muslim subjects lived in the Crusader states and were not necessarily forced to convert at swordpoint or wantonly killed (although both things happened). The behaviour of the Crusaders, while certainly brutal, was more acquisitive than destructive in its overall goals. They sacked cities, but their plans was more about ruling them than destroying them.
Their Muslim opponents were generally more tolerant, more cultured, more sophisticated (that's part of why they ultimately won I would say) but they also waged war frequently and brutally, and "tolerance" in a Medieval context was almost always a relative concept (well, the Middle Ages were more tolerant than Early Modernity in most of both Islamic and Christian societies anyway). Remarkably, in the Islamic world of the time at large, the Crusaders, and the Western Christians at large, were often seen as a nuisance, while the real threat was seen to be the Mongols - which, by the way, was largely correct, as the Mongols were in general FAR more destructive than the Crusaders ever were.
*Not that they would have understood such a notion as "Islamic civilization".