This is based on opinion. Please don't list it on a work's trope example list.
Tearjerker / Jennifer's Body
Chip's death, and Needy's grief-stricken reaction to it.
Watching Needy going from a cheerful, typical teenager into a violent and angry person, thanks to all the traumatic experiences she goes through. It gets worse with a dash of Fridge Horror: at the end, Needy escapes from the psychiatric institution and brutally murders Low Shoulder in retaliation for what they did to Jennifer, but where can she go after that? She's now an escaped convict with multiple murders on her hands, so she's likely going to be spending the rest of her life on the run or in hiding.