People don't seem to get this -
Keith HATES Natalie... He absolutely abhors her.
Keith felt his entire life as if Natalie had everything and he had nothing. Then, not only did he have nothing, but he finds out that he's DYING on top of all of this.
He hates her. He hates her (he says at much at the end and he MEANS it!) and wants her, and likes her and loves her all in the same breath. He's ultra bitter towards her, and he wants to make her suffer. And since he's dying, he figures, "Why not? Why not do something crazy and make someone hurt..."
You can see how much he hates her repeatedly in the film... No one seems to get that. You can see several points in which he not only says things that are utterly cruel to her but the expressions on his face, etc... You can tell it from the first few moments in the movie... He's going to "HAVE FUN" with a girl instead of ask her out, she's "TOO GOOD FOR YOU" (all said in VERY bitter tones) This is why Al is worried about him as he's saying these things. He wants to completely tear her down and then leave her high and dry after she finds out that he's dead.
The problem is that in the end, after she discovers his little secret (and the fact that he is trying to work through his hate and have feelings for her), she pulls something that he didn't expect and ruins his plans. She brings the element of redemption into the mix.
The scene in which she is saying goodbye to him (the last scene between the two of them) you can still see that "hatred/confusion" for her that he has all over his face, and she pulls (what can only be likened to) a "Broadway/Phantom of the opera moment" when Christine Daae' kisses the Phantom and says, "God gave me courage to show you you are not alone..."
In this moment, (the moment where Natalie pulls him to her and says, "I'm staying with you until you leave," You can see his struggle... that's where he loses it, and that's where he changes for the rest of his life even though his life is short. You see that he's so stunned by this reaction to him that he even spends a few moments with his hands just hanging in the air before he drops the suitcase to hold her back. It's this action, (full redemption, full forgiveness, etc) that changes him.
This movie is about how she changes HIM in his final moments, not the other way around.
It's about a major backfire on a plan because he wanted to take someone down with him and instead all that she showed him was unconditional love which wasn't what he expected at all.
I think the plot is amazing.