IMHO, the person who actually pulls the trigger is more responsible for a death than someone who orders them to. Legally they are the same, but to my mind, the actual assassin is more responsible because they could chose not to pull the trigger. This is different in a military & war situation, though, as soldiers are not allowed to defy legal orders from their superiors. But we aren't talking about the military here.
Can you elaborate more on the policies and orders that Brian Thompson gave that directly caused so many deaths? Even one death?
Did UnitedHealthcare under his direction engineer some kind of virus and release it causing a global pandemic killing millions and then he denied those infected treatment or something?
Or did this policy say something like "we're not going to pay for organ transplants for elderly people with three other conditions who probably wouldn't survive the transplant surgery anyway"?
If there were evidence of UHC policies or orders from Brian Thompson that did substantially contribute or cause even a single death, he should (and I hope would) have been prosecuted. For manslaughter at the least, but possibly murder. But I assume such evidence doesn't exist. If it does exist, please tell me so I know. Has to be something verifiable, not just hand-wavy "it's well known" or the like. That's just a conspiracy theory.
A policy of "we're not going to pay out more than the maximum specified in the insurance contract" doesn't count, as no one would expect insurance to pay out more than the maximum.
Of course, even if Brian Thompson was guilty of such crimes as some of history's greatest butchers, it still doesn't make shooting him right. But I would concede that shooting someone who used health care to kill hundreds of human beings, was somehow immune from legal prosecution, and intended to keep doing it might be morally (though not legally) justifiable.