I share your disdain for the murderer and disgust with those who champion his action. I can sympathize with his frustration with his insurance provider but can only condemn his extreme response.
However, I don't see your cited examples of similar "villains" as truly parallel examples. Smokers chose to use the product that made them sick*, people who ingest the processed foods, sugary drinks, fat-filled and high cholesterol products choose to do so, and gun manufacturers make a legitimate product with legitimate uses, sometimes misused by some.
You might as well include car and truck makers whose products are driven into crowds by crazed people, or those pilots who give sky diving lessons to people whose parachutes sometimes sadly fail.
Where does it all end? It won't. Stupid and evil people will always be with us. The only approach that might mitigate the massacres and dissuade some of the crazies is to let them see the consequences of the evil actions of others being held accountable, as we are now seeing with Mangione. His vocal and emotional cadre of supporters are misguided at best and complicit at worst. Acceptance of responsibility and condemnation of evil actions -not thoughts- are the way forward.
The fault, dear Brutus, is not in our stars but in our selves.
*The lies and concealments of the manufactures have been uncovered and dealt with in numerous and chastening lawsuits and prosecutions.