Sasori suddenly turned good and allowed his grandma to kill him, despite having been a ruthless killer before, having destroyed a nation and turned hundreds of humans into mindless puppets.
Kakuzu was taken out by a clone feint after having had around 80 years of experience as a Shinobi.
Itachi was killed by some random disease. [Of course, he'd still have died without it, but I'm just saying]
Deidara decided to kill himself despite his target having been severely weakened, going in for a complete overkill which ended up failing.
Hidan was tricked into killing his own partner, and was then blown up by a kid. [An intelligent kid, yes, but for a shinobi of his caliber, that's pretty pathetic]
Zetsu was killed in a random attack by Sasuke, the other was chopped by Chojuro.
Nagato decided to kill himself after hearing words from a book, despite already having heard the essence of the book itself, neglecting it's ways. The man who slaughtered even infants was turned good because he was quoted in a book.
The only ones who's death wasn't ridiculous was Kisame, Konan, and possibly Itachi, depends on how you look at things.