After Luigi Mangione made the difficult decision to undergo spinal surgery last year for chronic back pain, he became a proponent of the procedure that changed his life for the better.He repeatedly posted on Reddit about his recovery and offered words of encouragement for people with similar conditions, telling them to push back against doctors who suggested they had to live with pain.But notably absent from the posts are explicit concerns about corporate greed in the health insurance industry. Those appear to have surfaced only later: in a handwritten note found after Mangione was detained as a suspect in the killing of UnitedHealthcare CEO Brian Thompson.That short document references “parasites” in the health care system and laments the power and profits of health insurers, according to law enforcement officials. During his first public words since his arrest in Pennsylvania, Mangione emerged from a patrol car shouting about an “insult to …