when does keith start actually caring for natalie?

I personally believe it's when Keith drives to his secret spot across the lake from the bonfire where all the "cool kids" hang out while Natalie is hiding in the bed of the truck. While she's spilling every thought going through her head, the camera only focuses on Keith's face. You can tell he goes from annoyed to nearly crying and then all of the sudden he turns around and kisses her. I think that's the moment when he realized he had actual feelings for her instead of enjoying messing wiþh her life.

I agree that Keith always had some feelings for Natalie. But they were definitely amplified the night they went "bowling" and even more so when she unknowingly told him to turn into his secret spot.

Which would be worse, to live as a monster or to die as a good man?

this is one of my fav movies, had seen it so many times and just love the ending scene of natalia and keith .

i think he knows her, her feelings and much more as had an infatuation on her from his childhood

if you look at keith's car going away when she kisses her bf in the background and the way he behaves and mentions about it the next day
he was infatuated to her from his child hood but couldnt be with her
so with no further use of therapy he decides to be with her which is pretty much what every lover wants.... , but just to be and spend time with her
i think he has no intentions of messing with her life ,
we get the feeling from the beginning scene with the doctor and the ending scene when he tells her that he screwed her life

in the beginning scene he was casual without revealing his love for her as it was not necessary and tells him he just wants to have fun

in the ending scene he tells her that coz it appeared that way for everybody
including him and her,but we she dint buy that when she tells him to try again
the ending scene was a role play like some earlier scenes in canteen , chem lab
conf room , but was used by her to express that she loved him deeply irrespective of his condition and he wanted to hurt her with his words to test her whether she understood him properly and finds that she loves him as much as he does .

I have watched this movie over 50 times, and have watched every expression closely and every nuance. It is a very complex movie and Keith is a very complex character.
Keith was not necessarily in love with Natalie, but always had a crush on her since the 6th grade when he was Russian soldier #3 and she was the princess. That is why he chose her. He also hated her because he thought she was "TGFY" and also envied the life she had and also the life she would have in the future.
Remember that Keith had virtually no friends. He was a very lonely boy and also lost his mother at some point in his life. He secretly wished for a girl like Natalie but thought he could never get her. But, now that he was dying, he had nothing to lose. He then decided to try to strike up some type of relationship with her. Because he was so angry at his whole situation; he, at times, wanted to destroy her. However, deep down, he really wanted her to love him. The more he got to know her through their little encounters, the more conflicted he got because he was falling more and more in love with her. He was truly enjoying spending time with her, and she was too. I would go so far as to say that their little endeavors were probably the most fun he ever had in his entire life! Keith, however, always masked his feelings about just about everything, so he was always flip about emotions. THink about it, would an extremely lonely and outcast boy like Keith ever be able to resist the attentions of a girl like Natalie?

He started to actually realize he loved her the night she found his secret spot. If you remember, he said they were at a crossroads. Natalie, at that point, decided to stay with him, and they moved forward through the intersection. She then found his secret spot, and she really shocked him there. He realized there was alot to her and also that she actually had feelings for him. The kiss he gave her was real, he was not trying to be manipulative. At this point, he started to feel emotional pain because he was dying and knew there was no future with her. He then started to push her away because he is pretty incompetent at dealing with the painful emotions. That is why he pulled the stunt with the truck and the cliff the next day. He couldn't tolerate the emotions he was feeling. He wanted to push her away. He also (probably subconsciously) let her find his pills by giving her his jacket.

If you watch the scene where they make love, believe me, he is not play acting there. He was very obviously in love with her and had a lot of pent of feelings that were released. Watch the way they kiss---this is not a boy who is trying to hurt/manipulate her. This is a boy who is madly in love who is letting loose a lot of bottled up emotions. Afterwards, however, he again pushes her away because, he just cannot bear the pain of the relationship and the lack of a future. At this point he is also feeling a lot of guilt because he thinks she would have been better off without him and with Raff.

In the airport scene, he finally faces everything. He acknowledges his feelings. The reason she is not mad at him is that she ALREADY KNEW everything. She realized over time his motivations in the beginning, why he chose her as a lab partner, and why he was pushing her away. She understood. She just wanted him to acknowledge his love for her so that they could be together in his last days.

at the beginning keith had crush on her, but he wanted it just for fun.
i think keith started caring, at the scene when they both fought in the truck and then natalie almost left him, but keith didn't wanna leave, he stayed in the truck, and then the doctor and the kid came to talk to keith and natalie.
I am sure that's the moment.