First and foremost, autism is not mental illness. Autism is a developmental disorder. It is a disability that can affect many areas of development, including communication and social skills, as well as sensory processing and regulation. Some people with autism also have cognitive or intellectual disabilities, others have average or above average intelligence.
Autism is also a spectrum disorder, which means that each individual is affected differently. You may hear the terms "low functioning" and "high functioning" although those are not especially descriptive or accurate. Some people may, for example, be unable to speak and have severe anxiety but are quite eloquent writers and fully independent. Others may be verbal and able to tolerate noises and crowds but lack the executive functioning skills to live independently. This is a gross oversimplification meant only to illustrate that autistics are a diverse group that share some aspects of a diagnosis.
So what about autism and aggression? Generally, and certainly in Moe's case, aggression is a form of communication. It is sometimes an impulsive reaction to an external stimulus that might bother him, such as the dog barking, or an internal feeling of emotional dis-regulation. Primarily, however, because Moe cannot speak, it is a way for him to express frustration in a moment. He is telling us something, whether it is that he doesn't want to sit in circle time anymore, isn't feeling well, doesn't want his diaper changed, or just wants to be left alone. We are working hard to help him communicate those needs in other ways, but it is sometimes frustrating for him when we aren't getting what he's telling us.
Sometimes, Moe will do something that appears aggressive but isn't intended to be. For example, a few minutes ago, he got very excited and was running around a little wildly. He ran up to his little sister Jelly and pulled her hair. This was not intended to hurt her - it was just an impulse. He was smiling and giggling and didn't know how else to interact. Moe might bite because the deep pressure is a way to calm himself down when he's over stimulated or upset. We have done a pretty good job of teaching him to bite a chewy toy instead of a person or himself.
What Moe's aggression isn't, however, is pre-meditated. It is purely in the moment, usually easily mitigated when we are paying attention to what he is telling us, or at least redirected to something more appropriate. He can, for example, crash his body onto a large pillow, but not onto the dog.
Could someone with autism perform a pre-meditated act of violence? Of course. Autistic people may have other forms of mental illness or can be otherwise driven to violence in the same way that a neurotypical person could, though people with autism and other disabilities are much more likely to be victims of violence.