Friday the 13th, Part 3: 3D
Chris' (Dana Kimmell's) big secret of the Higgins's Haven is that she was nearly killed by Jason two years prior. When everyone is killed but her, she realizes its Jason (Richard Brooker) who is killing her friends and gets a burst of adrenalin and axes Jason in the head. She is institutionalized in the end.
Thanks, Jake M!
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