What are we supposed to take from this movie *spoilers*

I'm not being purposefully obtuse. Was this supposed to be romantic, or not or...

First of all I see so many posts about how much he hated Natalie and all that. I must have gone in with a totally different perception, because I thought the whole time he obviously had a crush on her. Even when he was saying "to have fun with" in the beginning, I wasn't convinced. I was like okay obvs he's saying that but obvs he's going to fall for her. Maybe he had some resentment that she didn't notice him in the play, or whatever, and so there was some bitterness...but for the most part he was just hitting on her hard.

So the whole..."oh I was trying to ruin your life"...that had gone over my head...I didn't see that throughout the movie. He was kind of an f-ed up person, but if ruining her life was his only goal... I really feel he could have found much better and more harmful ways to do it. Unless it was just the woo-bang-goodbye that he was going for.... But he seemed all bummed there too after she left the truck, in a "I must push you away for your own good" type way.

I don't know. I think he liked her, told himself he was doing it just for fun, but really did like her... and was being all weird and defensive and acting douchey later because he knew he was going to die... I had also thought, before I learned he was terminally ill, that he had some abandonment issues by the way he acted (confirmed, I thought, when I saw how his own mom had died)... and that since he was unstable, once he got close to her he shut down because closeness equals pain in his life, and he's been hurt so then he gets some backwards pleasure out of hurting someone he cares about, although he does care...or whatever... Because I really just thought he was emotionally disturbed, abused, in foster care...something... and that'd be the "twist" (that he was in a mental hospital those two weeks)...not cancer.

Either way, still not sure what we're supposed to take from it. Her life did end up ruined because of him. Then she's driving his truck and all that at the end, carrying out his goal etc...are we supposed to feel heartwarmed? That her life was forevermore changed/altered in a good way (because I'm not convinced of that)...or...what? Just what lol?

I felt like I was with this movie for the first half, but then it fell apart and never really made full sense afterward, as far as character motivations (him and her).