Aborrecerla means "to loathe her" or "to detest her." It's a strong and dramatic verb that goes beyond simply disliking someone.
In this song, the singer grapples with intense conflicting emotions. He questions why he's singing about a woman he "should loathe with all his heart," highlighting the deep internal struggle between his lingering affection and the pain she caused him. This powerful word captures the song's central conflict.
Un Montón De Estrellas is about the ache of loving someone who does not love you back the same way.
The singer admits he has tried and failed to forget her. Even though he knows she takes advantage of how much he cares, and that she has only ever kept him around for entertainment, he cannot let her go. He calls himself a fool for love, someone who keeps losing but never finds the strength to walk away.
By the end he confesses just how far he was willing to go for her: he would have climbed to the sky itself and brought down a montón de estrellas, a whole pile of stars, just to give them to her.