The only reason in my mind to view Mangione unfavorably is that vigilante justice, however justified, does not result in a civilization anyone would want to live in. It's the same reason we can't just go around killing everyone in the Epstein files. It's not that they don't deserve it, it's that the resulting corrosion destroys the entire system, including the good parts.

If Mangione had killed the UHC CEO by being his insurer and denying his claim for cancer treatment that his doctor said he needed, nobody would be saying anything.