whether or not the land-holding group known as ISIS, ISIL, or IS, is indeed evil

They obviously do not believe themselves to be the bad guys.
By our values they are, but I would say they are in absolute terms too.
Cruelty is evil and they revel in it.
Taking women and girls as sexual slaves is condoned in the Koran so that is less clear cut absolute evil.
Likewise killing people because of their nationality or faith.

Good or evil, they declare us to be their enemies and threaten us so of course we must resist them.