Are you aware that Mangione had no relationship with United Healthcare at all? He simply chose someone as an avatar for the system, a repository for his vague resentment. "This guy is rich, good enough."
In a universal system he'd simply be killing some other rich guy, or maybe bureaucrats.
I wouldn’t necessarily call them “bad reasons”. You can’t deny people essential healthcare using faulty AI while lobbying against universal healthcare in a country full of guns and expect to not to ever see any consequences. The irony is universal healthcare would have saved Brian Thompson’s life.