Death Note dammit.
Even if he wasn't the main character

L was a major character. After he died, things went downhill for me. I don't think I even read the entire thing, just skimmed to the end. Didn't like that either ! when Light died. I liked the false ending better.

When they're supposed to.

You can't just arbitrarily decide that THIS when the character is going to die, you have to consider the story in question, how it flows, and when the time is right for such a thing to occur. There's way too many variables to just pick out one specific time, because each plot has different times for which such a thing will work.

That's why all my books end with a side story or two that has nothing to do with the rest of the plot in the book and is just misc world building, usually in the form of some piece of folklore or history alluded to at some point in the series, but would've been disruptive to break up the plot JUST for a detailed bit of exposition about a minor, inconsequential detail, especially since I absolutely hate "as you know" situations, unless there's a legitimate plot purpose for it.

Ditto. There's strength and pride behind killing off a beloved character if it's done for the sake of the story, and as much as I've hated seeing some people die suddenly, it's also pretty damn "awesome" a lot of the times.

... I don't like my main characters to disappear (or die, I hate tragedies), which means right at or before the beginning - because that way death still doesn't get rid of them (otherwise they can't be the main character). Is Yu Yu Hakusho too old to register in people's minds anymore?

This reminds me of the MCs of the Dark/Light manga written by the MCs of Bakuman.