Analysis / Yandere

A possible origin of the Yandere trope

Yandere may have emerged as a deconstruction of the Yamato Nadeshiko archetype: a eternally serene woman who always puts the needs of her family and husband before her own. A yandere is all of this driven to the logical extreme and then examined realistically: a sentient individual put in such position would invariably experience enormous strain. Sooner or later she would snap at the slightest provocation. Ironically, it is this realistic take that makes a yandere so endearing to the audiences, often even more so than the traditional Yamato Nadeshiko. (Granted, there would realistically be a wide range of possible responses other than going Axe Crazy, but this one is definitely the most entertaining to watch!). Compare with the Violently Protective Girlfriend who can become Axe Crazy when the love interest is in danger but is otherwise MUCH more stable mentally. See also Living Emotional Crutch for a reason why she goes from affectionate to murderous.

Why Yandere can be perceived as attractive

Someone who is crazy for you is someone who is crazy for you.

Interestingly, these types of characters can still be perceived as attractive if they can build up enough tragic audience sympathy, despite the argument that many originally become popular by being subversions of wishy-washy female characters until the fandom ended up loving them in the typical way (what sweethearts!). Sometimes this can play out in story, if the Love Interest ever starts to rethink his relationship with this girl, she might threaten to harm him or herself if he ever leaves her. This threat might be explicit, but often is more subtle.

There is a guide how to win a yandere and live with her. Be warned, once she falls for you (or rather, YOU for HER) there is no turning back. God help you if you get stuck in a Psychotic Love Triangle. We all know how those go down.

The Trouble with Yanderes

A main problem with a Yandere is that a poorly-written Yandere is easy Scrappy bait. As outlandish as the trope can be, Yanderes can still be likable as long as they have elements of sympathy with them, but alas, there are a few that lack that key redemptive trait. A Yandere written to be unlikable is more than likely to be a villain of some caliber, planning to cause harm to a more level-headed character in a fit of blind fury simply because their dream relationship fell apart. Another bad sign for a Yandere is if he or she suffers from Erotomania, and delusionally believes that the person they love is in love with them when in reality, the person is either indifferent or wants nothing to do with them. Unrequited Yandere attraction never ends well, and can be very ugly if the Yandere goes off the deep end.

It is of the best interest for a good story, and all-around just to be taken seriously, to not have any main villain be motivated by Yandere leanings, and instead focus on it rather being a secondary character attribute or malady.
http://tvtropes.org/pmwiki/pmwiki.php/Analysis/Yandere