Some crusades were not even against Islam.
For example, The Crusade of Aragón was declared by Pope Martin IV against King Peter III of Aragon in 1284 and 1285. Peter was supporting the anti-Angevin forces in Sicily following the Sicilian Vespers, and the papacy supported Charles of Anjou.
Pope Boniface VIII proclaimed a crusade against Frederick III of Sicily, the younger brother of Peter, in 1298, but was unable to prevent Frederick’s crowning and recognition as King of Sicily.
Also, In April 1487, Pope Innocent VIII called a crusade against the Waldensian heretics of Savoy, the Piedmont, and the Dauphiné in southern France and northern Italy. The only efforts actually undertaken were against heretics in the Dauphiné, and resulted in little change.
And then there was the Albigensian Crusade. The Albi believed everything that tied humanity to the world of the flesh was evil, including vaginal (but not anal or oral) sex, and even eating. There were never too many true believers (perfecti) because their asceticism meant certain death at a very young age with no heirs. But they also attracted a larger group of supporters who were promised propitiation for their sins by the perfecti.
The Catholic Church expected an easy war to suppress the perfecti and “liberate” those influenced by them, but was surprised by the counter-offensive, led by nominally Catholic kings. The response by the German mercenaries was so brutal that according to legend which is certainly false, but propagated because it seemed to represent the attitude, the German warrior supposedly instructed, “kill them all (Catholic allies of the Albi and the Albi themselves); Let God sort them out.” IN the end, 20,000 people were supposedly killed, although this is likely an exaggeration.