Can someone spoil me on the ending/what I missed? (SPOILERS)

Not a play by play, but she manipulates him into going on a picnic. They get there and she reveals she has forgotten the sandwiches. She offers him a cherry soda, claiming it has no alcohol. Next scene, they're sitting against a tree clearly intoxicated/ drugged. She whips out her little cutter box and tells him to prove his love he has to understand her better and the way to do that is to cut himself. He begins, stating how he can't even feel it. As he loses consciousness, she takes her father's ring off of him and tells him he's just like all the others. That someone will find him. As she's getting ready to leave, his mom and a detective come up on the scene. They had tracked them by getting the cab company who took the kids there to tell them where he drove them.
The mom wrestles her off of her son. This was a hard scene for me, I have teens.
She keeps fighting the mom, and the mom ends up on top of her strangling her. The detective pulls the mom off just as the ambulance arrives. No pulse. They put the shocker thingies on his chest and revive him. It ends with the son and mom talking about his dad's upcoming wedding. All is well. The final scene is the psycho in the psych ward. A handsome young dark haired kid who works there brings her lunch. He sees she's holding a pic of her and her mother (the same one from her cutter box). He says "Is that you?" and she says "Yes, and my Mommy." then unfolds the pic to reveal Daddy "When he was sweet".
Daddy looks a lot like all the boys who became her victims.
Roll credits.

Sadly, at the end of the movie Grant's tinkle gets cut off!!!

And now his tinker totters!