I think most fanatics think this way regardless of culture, religion, time period, or location. It's not a colonialism thing, it's a religion thing. You can make the invisible people who are always right say anything you want to, and a lot of people make them say "Your cause is just, your enemies are ontological evil, and you look great in that necklace made of severed ears".

Religion basically gives you an invisible friend who tells you everything you do is the right thing to do, and since you believe your invisible friend cannot ever be wrong you can do whatever seems like a good idea to you. Which is fine if you're a morally centered, decent person, and really bad if you're a bloody minded monster.