Diablos literally translates to "devils", but in this song, it's used as a dramatic intensifier in questions, just like saying "the hell" or "on earth" in English.
Becky G repeatedly asks, "¿Cómo diablos...?" (How the hell...?) and "¿Por qué diablos...?" (Why the hell...?), demanding answers about why her relationship ended. Learning this word is a great way to understand how Spanish speakers add frustration and emphasis to their questions in everyday conversation.
Becky G is going through a massive heartbreak in this song, and she is not holding back. She opens by saying she has a heartache so bad that "pa' olvidarte / No existen pastillas" [to forget you / there are no pills]. She feels like she's been diagnosed with a lifelong lovesickness. She's completely confused about how her ex could leave, asking him directly, "¿Por qué diablos fue que tú te fuiste?" [Why the hell did you leave?] especially since she gave him more than he ever asked for.
But this isn't just a sad song. She flips the script from feeling down to feeling defiant. She's tired of being miserable while he seems happy. She decides, "pa' la mierda la melancolía" [to hell with melancholy] and starts to reclaim her power. She knows her own worth, declaring, "del uno al diez yo soy un fuckin' cien" [from one to ten, I'm a fuckin' hundred]. The final twist? She calls him out for being unfaithful and says that while they might meet again, he'll never be her official man again because he's an "hijueputa infiel" [unfaithful son of a bitch]. It's a powerful and blunt journey from pain to self-worth.