In the tv industry, we refer to this as Pulling a Joss Whedon. If they mean something to you, their days are numbered. After it happens, you wonder how in the hell you didn't realize that a certain character was going to meet an untimely end. There were clues all along, it just makes dramatic sense...regardless, our immediate reaction is to stare dumbly at the screen, while a single tear runs down our collective cheek. Why do they do this to us? Are showrunners inherently evil? Were they not hugged enough as children? Whatever the reason, they're undeniably sadistic. But it's hard to get too upset, when it creates such fantastic entertainment. It goes without saying, but I'll say it anyway: THERE ARE SPOILERS HERE. Big, honking ones. Read at your own risk.
10. Fred - Angel
Everyone loved Fred. I will stand on the rooftops and proclaim for all to hear that it is nigh on impossible for anyone to dislike Fred. That's why it was so horrifying when we saw her infected with a mysterious substance, and quickly decay until she was just an empty shell for the goddess Illyria to inhabit. It was textbook Whedon -- she and Wesley had finally gotten together, they were finally happy, and then this. What makes it absolutely the worst, though, is that Angel Investigations had to grieve the death of their friend while some alien thing walked around in her carcass. It must have been terrible for poor Wes to have to see Illyria everyday, knowing that even though it look likes Fred she's gone and she's never coming back. You're a sick man, Joss Whedon. Seek help.