There is some surprise in the ending simply because Pancho has continually asserted that he is going to kill the man who took Rosa's life. Pancho's entire focus has been to exact revenge for Rosa's death. There is some surprise when he relents in his mission. However, given his maturation and the exposure to different notions of life and how to live it, it makes sense that he would not kill Bobby.
Through his experiences with D.Q. and Marisol, Pancho has understood a new construction of being in the world. The kiss he shares with Marisol is one that is “long enough for him to see the future.” This is something that Pancho had lacked throughout the narrative. A vision of the future is something he now possesses. In his final entry into Rosa's diary, Pancho speaks to his sister and suggests that "After you died I didn’t care much for life. Now I think we need to take care of it." It is in this light where Pancho has changed and thus the ending of not killing Bobby does make some sense. It is surprising in so far that it denies his stated mission, but given the context in which new understandings about life has emerged, the ending is logically consistent and not entirely surprising.