It's been a day, and it's still bothering me.

Okay, besides the ending being rushed, it just felt incomplete and unsatisfying. What's really funny is you could make it more complete, and still have basically the exact same ending. Here's what you do.
At the point where Damien comes to save Ian and finds his sister possibly dead, you DONT have him strangle Ian and take the fall. That was stupid. It was self defense. What you do is have the sister get caught for trying to murder Ian. She still gets hit like before and whether she lives or dies doesn't matter, but I'd have her live (so she can go to prison, and/or spend life in a mental hospital). Have the police search her place, and find a diary where she brags about the things she has done. It should mention the cat and the butler (the butler deserves justice!). Naturally it should include her plans for harming Ian. You could even add them finger print checking the pictures she left in Damien's place. They won't have Damien's finger prints, but they will have hers! She gets caught; Damien's reputation is restored (and maybe he can stop taking that medicine, unsure, and not important). Now Ian has been in and out of conciousness, but he was able to say the sister did it. Then he's unconscious for a while, wakes up, Damien is gone, ending proceeds as before. Add some extras explaining what happened, and why Damian disappered for years. We know part of it is him being silly, but the evil sister's diary would likely reveal he was romantically involved with Ian, which I doubt his family will like. But they'll also be feeling remorseful/guilty because of how they treated Damien in the past, which will make Ian and Damien's romance go smoother in the long run.
THIS is how the story should have went and ended. And I'm just going to brainwash myself into thinking that's how it went. From now on, my version is canon! (︶︿︶)＝凸