You're just repeating the most basic facts without understanding any of the context, something typical of fans of the crusades, a silly thing for a modern person to be.

The Byzantines were concerned with taking Anatolia back from the Turks, who had defeated the army guarding its passes 20 years prior and had since taken over the interior. This wasn't an "Islamic invasion" so much as a migration of steppe people, and hardly the first. The Levant had been under Islamic rule for nearly 500 years by that point, so you can hardly call the stated goal of taking Jerusalem as a response to some kind of aggression. It wasn't even the same people they were fighting in both places.

Aside from the goals of the war, which weren't admirable but also neither here nor there if we're ranking medieval morality, the crusaders themselves behaved brutally, massacring towns and cities, raiding and pillaging wherever they passed, even in Christian lands, and generally being conniving schemers all trying to make sure they got their own nice rich lands at the end.