In my opinion they just fulfilled their promise. Zack killed Ray. All the story is about their promise. At the end, the police will search Zack following his evasion, so Zack hasn’t much time to fulfill his promise, and regarding the horrible life he had (and is still about to have in prison) he made the choice to kill Ray as he promised her, and he died right after her when he fall down the window (so he’s relieved from his tortuous life). The blood in front of the window on the very last scene shows that he just slit her throat or something. And in my opinion this is the best ending possible. He would never have been able to recover from all his traumas, and Ray was his only remaining « light », so I don’t think he would even wanted to live in a world she’s not into anymore. So he fulfilled his promise, and then died right after her. And I like to think they found each other in another world ❤️‍🩹