He took out his pain/meds on the health care executive whose company was rejecting many claims, made a gun on the computer and shot and killed the late Mr. Thompson.
In the process he leaves all kinds of evidence and suggests that he is copycatting yet another nutcase who killed people leaving a manifesto.
That's what we are supposed to believe and what will likely convict Mangione.Who would not buy that.
Is that all there is to this? We may never know.
Luigi is showing up everywhere “Black Cat” hackers are extorting millions in ransom and stealing millions of health records.

https://www.state.gov/rewards-for-justice-reward-offer-for-information-on-alphv-blackcat-linked-cyber-actors-targeting-u-s-critical-infrastructure/
The reward for tracing Luigi Magione’s Black Cat personas, like Mark Rosario, is much better than the meager $50K that the FBI has already offered for the promise to turn in the United Healthcare killer in New York City, Altoona, Pennsylvania.
The Black Cat reward is ten million dollars, and the terms for collecting it are much more lenient.