I'm guessing in his pathetic and selfish thinking he thought if he got rid of the person that got in between Hotaru and him that they would get back together. Of course, he can't understand Hotaru's deep love for her sister and that her death would only drive her farther away from him. After all, it would be hard for anyone to pursue a relationship you think your sister killed herself over. This guy though, he doesn't truly love Hotaru, love doesn't act this way. He's plain obsessed and no different from a fanatic or stalker.
I think though as a character, he perfectly contrasts Shirota. While Hiroto has always acted with his own feelings in mind, even to the detriment of others, Shirota has done everything, even being willing to sacrifice his own happiness, for the sake of Miyako. Shirota went back in time to save someone's life, Hiroto went back to save his own feelings.