The thing is, whoever did it was pretty smart about much of the crime, which is why the story of how Luigi was caught and what they say he had on is person seems so odd relative to the behavior the previous week.

If it was Luigi, he was pretty disciplined and smart before, during and immediately after the crime with the only leak being showing his face at the hostel (which he may have been required to do when he gave them ID).

If he did do it and really had all that evidence on him, maybe he wanted to get caught? It seems crazy that anyone would hang onto a 3d printed gun when it would be pretty easy to destroy and dispose of it in multiple pieces and locations.

I’ve said it before but I truly believe that Luigi would still be free if he had plucked his eyebrows, regardless of whether he did it.

Edit to add, maybe he thought he’d get shot or captured at the scene and didn’t think too far beyond the immediate getaway? And with the stress of his picture up all over the news and social media, he kinda wasn’t thinking straight?

A slight variation on your second scenario is parallel construction.
I don't know if Luigi's the shooter or not, but if he did it, there's a decent chance whatever is presented as evidence has absolutely nothing to do with how they actually caught the guy.

Luigi did it, and he really was that stupid.

Given that he was valedictorian at his school, I find that unlikely to be the option.

I think you're missing a 4th option:

• He did it, and believed it not worth the anxiety/suffering of living the rest of his life as a wanted man, so did what he could to get away from the immediate scene but ultimately turn himself in so to speak.

And there are almost certainly more nuanced options out there.

Some criminals are really that stupid,

A lot of criminals are remarkably stupid. It's fun to imagine criminal masterminds making up a lot of their plans with meticulous planning only to be foiled by the heroes. But in real life most crooks are fairly dumb. Even intelligent people who do crimes can make mistakes that come off as remarkably stupid to people learning about them.

I watched a lot of episode of Forensics Files and other shows that document cases that were solved by slim margins. Some criminals on the show really did almost get away with it if it wasn`t for one tiny mistake. In other cases, however, they got caught ultimately because they left behind something that absolutely would have lead to their capture.

Like in one case there was a mugger who killed a college girl while she walking home. The area she walked through was wooded and dark. The episode documents how DNA was used after it had advanced sufficiently to get him and they had to track him down to get a good sample. But there is something absolutely critical that the show didn't point out: He got on police radar because he dropped a bag of his that had a medicine bottle with his name on it. If he just picked up his bag the police would have had nothing to go by at all.

The other problem is that planning a murder like the one for Brian Thompson and getting away with it is hard as balls. Other than the fact that there are cameras everywhere. The guy needed to be at the right place and the right time and be there for only a minimal amount of time (to not draw undue attention) and he also had to be sure that there weren't that many police around so police response times would be extended. I think he did it at the time because he also knew that the police change shifts so there are fewer beat cops around.

Also the silencer obviously doesn't make the gun 100% quiet, but it was done to make it quiet enough to not set off the shotspotter towers. Those towers are more and more common in many cities so that when a criminal fires a gun, they can quickly triangulate where the shot was fired from. This would give away his location very quickly and minimize police response time.

It is meticulously planned. But there could be any number of things that could have gone wrong. The gun could have simply not fired at all. There could have been a few cops working extra hours nearby and ran after him quickly. Someone could have jumped at him to stop him right away. Lots and lots of factors go into it.

Even some people said that why was he using a hostel (which has security cameras and other witnesses) when he could have rented an AirBnB apartment. Some places rent rooms and apartments for people and those people never actually meet the host, meaning there is no one who would see you.

As for how to pay for them... the guy could have used a prepaid cash card, one bought and charged with cash. It cannot be traced back to you easily, if at all.

Fourth option, he's taking the fall for a love interest.

The one person interviewed by media who had spoken with Luigi said the note (manifesto/confession) wasn't at the standard of Luigi's other writing, his style, his philosophy, his reading