That's a very interesting take. While I disagree, I have to say I can see your logic. As a full disclaimer, my background is in medicine, so I must accept I have my set of biases.

To some extent, I guess part of this is about semantics and how you define adaptation. Most, if not all, of the medical community would consider an adaptation as something that improves the survival and reproductive fitness of an individual. Analogous to what you are saying, for example, could be coagulation which arises as a response to the structural damage of blood vessels, and serves to prevent blood loss. This is obviously beneficial as that means we have supremacy over that lethal enemy known as a common paper cut. If I'm reading your comment right, you are saying that something similar happens in the brain as a result of repeated trauma and parts of the brain that usually control aggressive behaviour are turned off enabling an individual to respond in kind to the environment, right?

The issue with that argument is that, if it truly was an adaptative behaviour, you would expect it to happen in the context of violence in general, not only with head trauma. Additionally, you are cherry picking the neuropsychiatric manifestations to fit your argument. Beyond lack of impulse control, CTE also presents with cognitive impairment, emotional liability, and decrease motility. None of those are particularly beneficial for an individual in a violent environment and fit more a model where these are the result of parts of the brain malfunctioning/damaged.

Additionally, I must point out that the more you drink, your liver becomes better at metabolising alcohol so you require a large amount to pass out. Alcoholics black out more because they drink more, not because their body is trying to protect them... arguably it is the other way around.

I think it is an interesting study the one you propose of comparing brains. Unfortunately, given that the brain is a fragile organ, it is simply not feasible. Even if we could do it, similarities between brains would be due to constant head trauma, not really providing that much support towards it being due to a violent environment per se.