I’m writing a slasher with a mystery non-supernatural killer. Similar to Scream but more mysterious

no split up, no flashback, no bully origin

I was watching some French wodunit miniseries on Netflix the other day with my dad and he figured out who the killer was before I did. He simply said "there's always one character who has no discernible purpose or role in the story, who has no real reason to be there, or whose motivations are unclear. It's always that guy."
I guess don't do that. Don't make your killer just someone who is on screen for no reason because he's the killer despite the fact that nobody knows it. Make him genuinely seem like one of the protagonists and make sure he has a purpose.

heavy rain did that
if the killer is a protagonist who doesn't appear to be a killer (as they obviously wouldn't, given it's a mystery), the reveal is likely to seem contrived and at worst, like a complete asspull where the viewer was literally lied to in a way that defeats the purpose of giving out clues

Opinions differ I suppose. I think Heavy Rain was a boring mess to begin with and the twist was one of the few neat aspects of it. It just handled it wrong, like many things.
I don't think you should compare a video game to a movie either. Heavy Rain made you play as the guy who later turned out to the the killer, literally putting you in his shoes. There's a disconnect there. You think: "I'm this character. I would never kill kids. So neither would this character." And when it turns out he does you feel betrayed by the storytelling in an insatisfactory way.
Meanwhile, a movie is just a play. You don't actually act in it yourself, unlike a game.

the suspension of disbelief related to this main character secretly being a serial killer would've made more sense if there weren't actual instances where it would've been literally impossible for him to have killed/done other criminal acts in the time frame presented to the viewer, even relative to other characters being mere feet away during them
Especially in the context that the game expects you to accept the possibility that a separate protagonist might be the murderer for maybe 1/3 of it, and yet another character is pursuing them as if they were through most of the story
Not to mention the game is focused more on the static elements of the story than the aspects that can change, or even the gameplay itself

My point is, the exercise of having a leading character be the killer after spending time establishing that they probably aren't is just going to seem cheap, unless it's masterfully done in such a way that I can't recall a movie or anything else ever doing long-term, or it's a character of relative unimportance compared to the handful of major characters

The more time you spend with them, the harder it becomes to obfuscate their true intentions in a "fair" way, if you intend for your mystery to be solvable with the right amount of paying attention and logical leaps. Even an example like Scream (the first one) used in this thread has a severe problem with one killer being too underwritten for someone to ever guess he gave enough of a shit about his dead mom to murder a bunch of people, while the other was just a psycho the whole time and was doing it for fun and attention.

Make your killer target only women with gigantic asses and having plenty of shots focusing on said asses. the killer is Irish

Pretty much right on the money here.
I don't believe there needed to be a twist. Let the player know they're playing as the killer, and then switch seats and play as the dad trying to save his son from the killer. Make it a cat an mouse game between the two characters that the player both controls as the story demands it. The suspension of disbelief and ability to immerse will let people switch between these two guys easily; maybe they'll even start identifying with the killer because they're having fun playing as him which adds another element of duality to the story and opens up the inevitable implication of "he's a fucked up serial killer but he's not so different from you and i, really makes you think."

>Sir! It's the third victim in the gigantic ass killings!
>That does it! Round up every Connor, Mac and ginger looking freak in town!

Is the knife his penis?

The android one did the multiple and opposing protagonists a lot better (aside from the useless chick robot), where you could angle the cop robot and the black robot's storylines in such a way that they ended up having a fight to the death at the end where you had to pick one to control and kill the other (or get killed by them if you fucked up).

he has a particular set of skills

Killer mercilessly rids a town of white women. The killer is revealed to be Marie Kondo and a group of other cute small Japanese women, working together with the white women's husbands and boyfriends. They get away with it and live happily ever after.

Based

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Like character A says he’ll be driving to get groceries, character B is sitting in his room and suddenly he hears a break in. Character B is prepared for an attack and investigates only to find Character A lifeless from the gas in the car.

have an unsimulated female urination and/or defecation scene