As I've already mentioned, while Shuffle may have introduced the concept of the yandere to a wider audience, School Days and its "nice boat" exploded it into wider anime fandom consciousness.
Its anime adaptation gained special notoriety in particular for what happened with its final episode. However given everything I've told you so far about yandere and the original visual novel's reputation, you can probably guess that it involves an axe murder (pretty much the yandere definition).
Their pupils dilate and become tiny, or alternatively their eyes could go completely dead, as though their soul has just disappeared in pure horror at what unfolded in front of them.
That, and the part where they start swinging their axe above their head while giggling and talking about how much they love you and want to make sure you never leave them by chopping your legs off.
When the term "yandere" was first used is buried somewhere on a Japanese internet forum, although we can at least assume it came after 2002 as that was when the term tsundere was first used.