1. Chris was never real. Just a literary device that helped end her writers block. This was hinted at by her writing in the notebook at the end about his death.
2. Chris was real, and the story he wrote was actually his murder confession, and he killed himself after leaving her apartment.
3. Chris left her apartment after not killing her because he realized she was deeply troubled and so she creates a new ending for him bc it's too hard to face the fact that he simply walks out of her life. I was thinking that from his perspective he's a freshman in college and some lady wants him to help her with assisted suicide. It might have been too much for him so he did the honorable thing and left her alive.
4. To take her word for everything and that Chris really did just drop off his novel, not kill her, and died.