That said, apparently his manifesto is fairly short and honestly sounds more like a confession than an actual political manifesto.

My point is more that usually when you hear about a killer having a manifesto you expect a lenghty diatribe about what they think is wrong with society and why they think what they did helps fix it - whether it's early 20th century "propaganda of the deed" anarchists, late 20th century "fall of the West" primitivists or early 21st century "race war" white supremacists and "new crusade" Christian nationalists. Of course for e.g. Islamist terrorists you don't even need a manifesto because everyone knows the cliff notes version already (Western imperialism, Islamic caliphate, blasphemy, etc). Instead this guy seems to have largely been upset with privatized healthcare, which is a common sentiment but rarely enough to motivate someone to pull off such an elaborate stunt.

That his manifesto is pretty rushed and incomplete does support the idea that he's more mentally unstable than genuinely "politically radicalized" though. The Christchurch shooter's manifesto for example was fairly incoherent and seemed more like an elaborate trolling attempt than a sophisticated political tract but clearly some effort went into it. Luigi's almost feels like a half-hearted homework assignment. I wouldn't be surprised if he quickly wrote it after the killing on a whim and didn't give it much thought before, which again would fit with my impression that he really focused on the first killing and didn't plan out much beyond that. As someone struggling with ADHD and autistic hyperfixation (not saying either of those apply to him), I can relate.