Bosley. Just to see what would have happened if the Angels had had to confront, you know, a real villain who killed one of their own. '70s Charlie's Angels vs. The Zodiac. I want it.
As for characters who should just die already...Mario. If he died, maybe Nintendo would actually, you know, make some new stuff. I'll put Link on that list too. I would have said Zelda but it occurs to me that, for a game called the Legend Of Zelda, Zelda doesn't actually do very much (with the exception of OoT), so killing her when the games are all about Link is kind of moot.
Also, it would be nice if Jean Grey/The Phoenix could actually stay dead for once. She dies so many times it's like she's got a spawn point. oO
As for characters who shouldn't have died...I would say The Wicked Witch of the West, since she fucking rocks, but since they've revived her in the form of a book and musical prequel, I guess that's already been done.
Who else? Umm...Oh, Fred from Angel. She was like the only character on that show I liked. And, for that matter, Wash in Serenity. You can't kill Wash! He's...He's WASH!