“I guess you didn't care, and I guess I liked that
And when I fell hard you took a step back”

Taylor Swift – I Knew You Were Trouble Lyrics on LyricsMode
She's saying that she likes the bad boy type and she probably thought it was a turn on about him not really caring. When things got hard or wrong for her, he wasn't there and kind of "vanished" while the problem was there. So in reality he didn't care about her in the first place.
She's saying that he didn't care about the problem, she liked that about him, and when it had to do with her, he walked away.