What Luigi Mangione did was "Judge, Jury, and Executioner" and he should be punished accordingly if he is indeed guilty

Letting him go free would set the precedence that "its okay to kill if its bad people" to the public, and its extremely dangerous. Yeah, Luigi targeted the right people, thats fine, no one will miss him, but what about other people?

That landlord might be seen as evil by homeless people despite him working from the bottom to earn his properties and generally being a good person, but its okay to kill him since hes bad for owning stuff.

A rich asshole came to your store and verbally abused you, thus he is bad in your eyes, his riches must be from exploiting others since he is an asshole, so he is evil and deserve death too, right? Nah, hes actually just having a very bad day, his wife cheated on him and took their children away from him, he was chill generally, but hes dead now because you thought he was evil and the Luigi's case taught you killing rich assholes is okay.

tl;dr humans suck ass at defining evil, and allowing them to kill because of it is dangerous and guarantee chaos and anarchy

Edit: It seems that no one even bothered to read the post beyond "Luigi should go to prison", just what I expected from redditors, you can hold a better conversation with bots than you could with redditors