For some fans, part of the appeal of anime is thinking about which characters would be relationship material if they were real. Some anime characters seem like they'd make terrible significant others, though. It could be because they treat their canon boyfriends or girlfriends like garbage; their cartoon relationships are truly disturbing. It could be because they're really insecure and willing to inflict violence on anyone who so much as looks at their crush. It could be because they're just really exhausting to be around.
One person's dream date is another person's nightmare, so you likely won't agree with all the selections here. You might even give some a chance despite all the red flags, but hey, no judgment.
Nobuyuki Sugou - Sword Art Online
Nobuyuki Sugou desperately wants a relationship with Asuna Yuuki, but he’s so utterly repulsive that she's uninterested. So is everyone else. Despite knowing full well that Yuuki can’t stand him, Sugou nearly tricks her father into allowing him to legally marry her while she’s in a coma.
He also traps Yuuki in a virtual gilded cage so that he can accost her whenever he wants. This is how he treats someone who he claims to care about, so it’s pretty clear why Sugou isn't boyfriend material. Also, by the end of Sword Art Online, he’s in prison, so there's that, too.
Light Yagami of Death Note is one of the worst possible people to end up in a relationship with. First of all, he'd only date someone that he could use and coerce. For example, he sees Misa Amane and Kiyomi Takada not as people, but as objects who he can manipulate into obedience. He'll laud his relationships with both women, claiming to care about them but in reality he happily uses the Death Note to make Takada burn herself alive when it proves convenient.
Light Yagami might be charming but having a genuine relationship with him is pretty much impossible. There's also a good chance that his significant other might end up dead.
Makoto Itou of School Days is infamous for being basically one of the worst boyfriends ever. The instant that he realizes that girls find him attractive, he immediately embarks on a odyssey of serial screwing. This could be okay if he were honest about it, but he's not. Instead, he lies to his girlfriends and makes promises of exclusivity that he has no intention to keep. He finally decides to be honest by making out with his new girl in front of his pregnant ex. Basically, Itou is a total scumbag.
That said, the girls he pursues aren't that great either, especially Sekai Saionji. She kills him and chops his head off. No amount of philandering and deadbeat behavior justifies decapitation.
Yuno Gasai of Future Diary is dateable but that's because you kind of have to date her if she's into you. When Gasai likes someone, she likes them to the point of madness and obsession.
For example, she stalks Yukiteru relentlessly, tracking his every movement with her magical diary and inserting herself into every crevice of his life whether she's wanted or not. Oh, and she also killed her own parents. While she did do it to escape their abuse, she keeps their corpses around and talks to them like they're still alive. That's kind of a lot to deal with.