It comes from the Lord of the Rings, that's just someone using it. I don't think it has anything specifically to do with Muslims. They use it to dehumanise, that's true. It mostly spread as use for Russian soldiers that were just rampaging around killing and torturing civilians, or for those shelling and killing civilians, ultimately - behaving rather inhumanly. I've only ever seen it referencing Russian combatants. Honestly, comparing your enemies to characters in a fantasy novel isn't exactly the worst slur in the world.

Soldiers almost always have a slur for enemy combatants, sadly, but honestly I don't blame them. Dehumanising your enemy is a sad reality of war.

I agree that us as bystanders should try not to use such slurs though. Ultimately, throwing slurs around doesn't look good even if the people you're throwing them at are committing atrocities and invading countries.

Edit: I agree it's racist, reminds me of WW2 soldiers's slurs like "jap", "krout" etc. Regardless of the original source, and ultimately it being rather mild compared to many, it's a racial slur and shouldn't be used.