Randolph Bell - The Resident
Bell's love for Hunter made him blind to all the evil things that she was doing. Hunter was murdering patients and giving them unnecessary treatments that could kill them. Bell refused to see reason until the end of Season 1 and unintentionally helped Hunter keep up her charade.
Taylor Hotchkiss - The Perfectionists
After Taylor's brother was killed, she was determined to bringer his murderer to justice. She ended up shooting Caitlin's boyfriend Jeremy, but it was later revealed that he was innocent.
Shane Walsh - The Walking Dead
Shane's love for his best friend's wife ended up being his ultimate downfall. Lori's refusal to be with him when her husband turned up alive led to a series of events that ended with him trying to kill Rick.
Jaime Lannister - Game of Thrones
In order to keep his affair with his Cersei a secret, Jamie pushed a child out of a window. He even admitted that he would kill anyone who tried to stop them from being together.
Zoe Benson - American Horror Story
When Zoe's romantic interest, Kyle dies, Madison helps Zoe put him back together limb by limb. But the new Kyle isn't the same person as the old one, and he ends up turning into a violent killer. Zoe falls in love with him while trying to keep him under control, and ends up protecting him instead of putting an end to everything.
Serena Joy - The Handmaid's Tale
Serena would do anything to have a child and be loved by one. She helped put the events in motion that created the Republic of Gilead and constantly abused June when her sexual assault wasn't resulting in a pregnancy.