Why did crusades attack Muslims?

Answer 1

There are a wide variety of reasons as to why Crusaders attacked Moslems.

1) Impeding Control of Jerusalem: The stated mission of the Crusades was to take back control of Jerusalem for Christendom. If they were trying to take it back, it follows logically that somebody else was in possession of it. As Moslems were in possession of Jerusalem and they did not wish to just give the city up, Crusaders attacked them.

2) Protecting Byzantium: The real reason for the Crusades was to prevent the Byzantine Empire from being completely overrun. The Seljuks, Ottomans, and Abbassids were all moving into Eastern Anatolia and threatening the survival of the Byzantine Empire. These states were all Moslem, so necessarily any policy to contain them militarily would require Moslem deaths.

3) Religious Discrimination: Many Christians saw Moslems as infidels who had knowingly rejected Christ and were thus fit to die. (This view has changed with time.)

For a more detailed analysis, see the link below.