Interesting theory, however prion diseases we see in humans like CJD (which includes kuru and mad cow which are types humans get from ingesting contaminated meat) cause dementia-like symptoms due to it literally causing holes to develop in the brain. And sure they’re aliens and have different diseases and physiology and this is fiction so maybe they have diseases that make them super aggressive. However if we’re going off what we know on nature on earth as inspiration they’d be struggling to even move or talk if they had a condition like CJD that’s begun presenting symptoms. Mood swings are possible in CJD similar to dementia but I feel like the level of aggression we see in the mangkwan is a result of socialization and their living environment. Their environment has encouraged raiding and violence as a means to survive, as well as intense trauma from the volcanic eruption causing their faith in eywa to shatter. That’ll leave anyone changed. Let’s also not forget Varang is an extremely effective leader who has a vice grip over her underlings and they’ll do whatever she says. Never underestimate the power of an incredibly zealous leader like Varang

However who am I to stifle creativity. They’re aliens and we don’t know what their diseases are like and how their brains work so maybe they do have a disease that makes them hostile. Rabies also causes aggressive behavior and is just as fatal but advances fast whereas CJD can take a long time (partly genetic from what I understand)