this article offers another explanation besides mental illness. i think it's worth a read, although it's rather shoddily put together and i don't necessarily agree with all of it.
i haven't seen any hard evidence (maybe i haven't been looking in the right place?) that holmes had serious mental illness...or autism, for that matter...but i have seen a lot of armchair diagnoses of both conditions in the news, which i find seriously troubling. i have a hard time believing someone can just "snap" when an execution like this requires so much pre-planning. this guy was ordering guns, ammo, and chemicals to make bombs for months before he did this. even loughner (who actually does have schizophrenia) did not exhibit anywhere near that level of pre-planning.
this blog post also compares some statistics on violence committed by people with disabilities to statistics on violence committed against people with disabilities. one of the things i find really scary is that people with mental illnesses and developmental disabilities (people who are neurodiverse) are much more likely to be harassed, arrested, or even killed by the police (this happens on a pretty regular basis) because we're suspected of being violent or assumed to be drunk or psychotic when we are in public spaces and we don't look "normal." i'm afraid that jumping to conclusions in cases like this can contribute to that phenomenon.