Well I think, that the two girls who started stealing the hearts were the guys exes whom he treated badly, and in return they rip out the girls he actually started falling for, his bloody hand is the one he used to rip out the two girls hearts, but since then, they'd cloaked their pain by inflicting it upon others, especially the guy. I really hope that makes sense. -- F.Y.I, I'm a dude.
I seems to me that all the girls on the left are the ones who's hearts the guy "ripped out". But while the Heart Thief was stealing these girls hearts, he wasn't expecting for someone to come up behind him and steal his heart instead.