That was the beginning of the end of P5 for me, the entire scenario is really kids picking fight for no reason, they randomly go to an exhibition and then overheard some people saying he copy work or something, and that's what they decide to kill him over?
Question: When was the last time you actually played Persona 5? Because that's not how it happens.
In Mementos, they encounter the Shadow of a man named Nakanohara, who'd given up on life because of what Madarame did to him, and called him out on it during and after the Phantom Thieves' battle against him. Nakanohara even posts on the Phansite to ask that Madarame's heart be changed later - which they don't find out about until after going to the exhibition, where Madarame just seems like a normal guy to them. It's not until they get the request that they start to research Madarame in detail, up to learning that he's messed up enough to have a Palace like Kamoshida did.
The Palace, in and of itself, is evidence that something is seriously wrong - at least by the logic of the teenagers, who A) are teenagers, and B) had already decided to continue changing hearts to improve society, because they don't trust anyone else to do it. So, given that they have a huge and "obvious" target in front of them, what reason did they have to turn it down?
Also, "kill" is very much the wrong word here. They don't kill people, and they don't want to - there was that whole thing in the first arc over whether it was worth the chance of killing Kamoshida or not, and they only went for it because they had no other choice. But since changing Kamoshida's heart
didn't kill him, now they know it doesn't have to be (and shouldn't be) part of their MO.
That's quite a drop from child molester (they eventually find out more, but that's long after they decided to target him) and seems just so small compared to all the other kind of big evil you can find in the world.
They don't have access to "the world". They can only deal with the bad people they actually know about, and Madarame is one of them - and bad enough to have a Palace, which is why they decide to take him on and dispel it.
Even keeping it to people they know personally, why not investigate the principal who didn't give a crap about the PE teacher? Or the rapist politician who get the MC throw into the bad school?
Principal Kobayakawa's a terrible person, sure, but he just doesn't have enough going on to make an interesting Palace out of. His problem isn't that he's "distorting" reality. As for the politician, see above: they don't know enough about him at that point to do anything about him.
Not understanding themes or subtext, I can excuse. Lord knows I'm pretty dense about that stuff a lot of the time. But you're getting the actual events of the story wrong, and that doesn't sit right with me.