The worst would-be player I've ever met was basically an actual neo-nazi. I met him at uni and he seemed okay if a bit of a bore, didn't say anything racist/homophobic/misogynist, just boring obsession with WH40K and the Space Wolves (which I now consider a "warning sign" lol), and he was part of a group of us nerds (I was regarded as the "cool guy who is good at talking to people" of the group which is hilarious/terrifying depending on how you look at it) who were planning to rent a house together for the second term and when he found out I was a D&D DM, he was really bugging me to run a game for us. Then he gradually revealed his true form, first with creepy misogyny (which we shot down and he shut up about), then eventually just straight-up "but the immigrants are destroying our way of life" racism. It was clear this was entirely from his parents (given all the "my dad says..." stuff), so I tried to see if he could be talked into sanity, but whilst he admitted he was wrong and was clearly somewhat ashamed of himself, the racism etc. was equally clearly reflexive, so we just ditched him and never spoke to him again.
In terms of actual players who actually got to play, I'm usually able to be pretty selective, so I haven't had too many bad ones, but I did have a non-fire-able player (a family friend and my brother's best friend) go through a terrible munchkin phase in his teenage years, which he thankfully fully recovered from. Still, for about 4-5 years he made nothing but basically munchkin characters.
And I do mean munchkin, not "power gamer" or min-maxer or w/e. Where a power-gamer or min-maxer will stick inside both the rules and the spirit of the rules, and often be otherwise a nice player and sometimes a good RPer, a munchkin will push the rules to absolute breaking point and use every exploit they can get away with, and will continually influence the game situation to show off how powerful their PC is, even at the expense of the other players having fun.
As he generally built combat monsters, he'd try to turn everything into combat. This was worse in some games than others. Like, in Vampire, he'd just get shut down, because he wasn't so dumb he wanted to die, so he couldn't start stuff with stronger vampires (yet desperately wanted to because Diablerie), and in other situations, he got threatened with mind-control or the like if he didn't behave, and he didn't want that either. Also you just couldn't be that great at fighting - a starting vampire is always going to be shredded by a werewolf, no matter how badass you think you are. So that was okay.
But Cyberpunk 2020 or Shadowrun or anything GURPS set in the modern day or future where you had more than like 150 points, or any number of other games? Oh boy. So many NPCs got shot in the face. So many plans got ruined because he couldn't not kill people. Even in Superhero games where he was supposed to be a good guy he just couldn't keep it under control, but I think I finally broke it when, as a DM, when his vigilante superhero (like an extra-violent Batman with a low-end version of Superman's powers) was stalking through the bad part of some city chasing a ninja-ish supervillain, and he heard someone coming up behind him. I knew the person coming up behind was a cop, and I expected him to turn and have to argue with them (because he'd been framed by said supervillain for a heist, but the cops were sympathetic to him and might buy that). But no. He specifically waits until the guy is really close behind him (on a blind corner), then kicks out backwards with maximum super-strength. I'm like "Uhhhh, are you sure? You don't wanna turn?!" and he's like "No, this time I'll surprise [supervillain]! He thinks he's so sneaky!" and hit attack hits, and he rolls high on damage, so a cop's dead body goes hurtling through the brick wall of a nearby house. SIGH. I admit the weeping grandma telling him he's "no hero" I said the house contained might have been laying it on a bit thick, but he seemed to revise a lot of his behaviours after that (I suspect getting past the "hormonal" stage of teenagerhood helped).