A few days ago I watched an anime about a series of child murders. The show was pretty solid except for the fact that I was able to correctly guess the culprit the moment the show established that they were going to be more than just a minor character. Was it because the show was really bad at foreshadowing? No, except for one scene where the protagonist casually mentions that the culprit could be someone he knows. Was the character behaving suspiciously? No, they were cooperative to the point of actively sabotaging the murder attempts. Then what did they do wrong to earn my suspicions right from the beginning?
Be a grown male authority figure with a more or less friendly relationship to the young protagonist without being related to them by blood. I'm relatively new to anime and visual novels*, and I've already seen characters fitting this description turn out to be villainous to varying degrees in about ten different titles (in one of them even twice within a few chapters).** Some of these guys are just being forced into doing something shady because of unfortunate circumstances while some them are psychopathic monsters because the writers liked themselves some shock value. Sometimes they are only minor villains or even red herrings, sometimes they are the big bad. What most of them have in common is that they at least pretend to be helping the protagonist out to the best of their abilities while secretly using the information they get from their conversations against them. I found only one example where the character in question is female, and even then the same twist happens another time with the male one being the major villain (#feminism I guess?).
I get why this trope is popular with writers. The culprit should be someone who has been introduced early on and appeared in a reasonable amount of scenes for their reveal not to come out of nowhere. They should be an adult to have the necessary physical, mental and financial capabilities to properly commit and cover up a crime. Having a close relationship with the protagonist is a good justification for giving them enough scenes in a story that mostly revolves around younger characters. Add to that the betrayal of the protagonist's trust and you have everything for a good dark twist.
So in theory "the nice adult is actually a murderer" can be a very effective trope if well implemented, it's just so overused it lost most of its its element of surprise, at least to me. I suspect that the writers of some of the titles I consumed were aware of this, but instead of not utilizing the trope they took the "let's add shock value by making the bad guy psychopatically evil" route, which is something that rarely turns out well, even if you add in discussions about the nature of psychopathy. I would really love to give examples of cases where I either really liked or really hated the implementation of this trope to add a bit more substance to my rambling. But I have no idea how to discuss plot twists when even mentioning the title of the game means spoiling them, so this will have to do.
*This twist isn't exclusive to Japanese media. For example two of the first four Harry Potter books utilize it and I played one well known western storytelling game that does as well.
**I even went through my vndb profile and made a list to confirm this. The reason I don't share it here is obviously spoilers.