Many people will say that 'death is not the end'. Despite what you may feel about this, it's certainly true within the world of comics. Ever since 'The Death of Superman' it's become somewhat of a running joke that there's only three characters that can ever stay dead in comics; Uncle Ben, Jason Todd and Bucky Barnes.
Then we got the Red Hood...and The Winter Solider, so there goes that notion. Fingers crossed we get a superhero version of Uncle Ben someday.
Though they have been thought of as untouchable the same can't be said about even some of the more popular characters. Dying in comics isn't new, but sometimes it's the way in which they kick the bucket that's really shocking. Their popularity may have brought them back from the dead, but that doesn't mean they didn't brave that final frontier; in one way or another.
With Infinity Wars coming up discussions are taking place across the globe as to who among the Avengers will die (because someone's obviously going to die). Therefore it seems only apt that we look back across the times in comics when the Avengers have been defeated, and thus required avenging themselves.