If his life was in danger, or he knew someone who was being denied claims that had died or their life was threatened, and he knew that the death of the CEO would lead to saving the lives of said victims, then its justifiable homicide.

But if his medical bills were paid for (they were), and he had no immediate relations being denied life-saving treatments (which he didn't), and he had no personal connection to United Healthcare (he was never insured by them), and he had no logical reason for believing that killing Brian Thompson would suddenly cause all insurance companies to massively improve rates of claim approval (he had no reason for believing this), then its not justifiable homicide: its just plain murder.