Maggie Greene (The Walking Dead)
If you had no idea that you've seen Maggie for possibly the last time, then you can blame the AMC hype machine for that. Because swallowed up in the whirlwind of emotions that followed Andrew Lincoln's final episode (but not death) was the relatively silent departure of Maggie. Her fate was left as open as Rick's, but for a character that was around since the second season, her departure warranted at least a little bit of fanfare.
Adam Milligan (Supernatural)
The half-brother of the Winchesters sole purpose for existing was apparently to prevent Dean from serving as Michael's vessel. After his and Sam's souls were used as punching bags by the feuding archangels, Dean chose to rescue Sam's soul and forget about the person who chose to offer himself up in Dean's stead. As it stands, his soul has been damaged beyond repair and is still in the cage.
Jesse Pinkman (Breaking Bad)
Jesse's misfortune was mostly of his own doing. The guy couldn't make a single good decision to save his life. But the way his fate was left open to interpretation as he drove off in a manic fit of relief and depression wasn't enough. Hopefully, the proposed spinoff movie will allow this unlucky chap to get the happy ending he so desperately needs.
Marcy Warton (Travelers)
Traveler 3569 was always on borrowed time once she was shipped into the body of a mentally impaired woman, but it didn't make the reboot she underwent any less heartbreaking. Technically, Marcy is still very much alive, but she is now without the abundant kindness and quirky mannerisms that endeared her to everyone around her.
Karen Jackson (Shameless)
Karen was a nightmare. She saw everyone as a means to an end. That trait was wiped out in the most brutal way by a jealousy-fueled Mandy Malkovich who ran her over with a car. Karen went into a coma and was then revived by the restoring power of oral sex. Afterward, she could barely piece together a full sentence or make new memories.
Jill Morgan (MacGyver)
Just when she was beginning to get more time on screen, they had to go and have Murdock, a thoroughly underwhelming villain, murder her offscreen. With Mac being one foot in one foot out at The Phoenix Foundation, the team was in need of someone who was ready to step in and fill his role. She seemed to be the natural option, but it was not to be.