Really? I'm sure they thought they were saving their souls. Or saving untold thousands (millions?) of souls from the depredations of these few evil pagans. Just like the Taliban created their totalitarian society purely out of love for their subjects: They didn't want their subjects (whom they loved dearly) to be damned for eternity.
In both cases there's that problematic belief that our lives in this - obviously existing - world is just an audition for being able to live in the "real" world to come - ironically the one world for which there is no evidence.
Anyway, this is a side-issue of a side-issue. The real side-issue is your contention that most Christians are Christians because there's something inherently true about Christianity.