I used Yuno as an example mainly because she's from the same series, and in my experience with these kinds of people, I very much doubt her more psychotic tendencies have any kind of bearing on their opinion of her.

Personal bias aside, I note that one of the other most popular characters in the series is Uryuu Minene, a psychotic female bomber who is also incredibly ruthless in getting her way, which is to win the Survival Game and become God. Pretty much everybody loves Minene, including these fans, so I really don't think Yuno being psychotic has much to do with it.

I also think that complaining about a character in Future Diary being crazy is kinda like complaining about loose women being in a brothel. This is a manga with a blind guy who dresses up in a whacked out superhero costume and blows himself up, a five year old who tries to kill Yukiteru and Yuno with a cyanide injected tomato, and a priestess who's largely lost it because the guy who killed her parents locked her in a cage and had her repeatedly raped.

All of that being said, I do note your point about Yuno and those other characters not being entirely even, and shall take it to heart.