Siervo translates to "serf" or "servant." It's an interesting and somewhat archaic word that isn't commonly found in modern song lyrics, making it stand out.
In the context of the song, the speaker uses "como un siervo" (like a serf) to describe a feeling of being completely devoted and perhaps even subservient in a past relationship. This word choice evokes a sense of deep commitment and perhaps a lack of agency, adding a unique emotional layer to the narrative of a lost love.
German Montero's "No Lo Hubieras Hecho" is a powerful song about a relationship ending, with the singer telling his former love to leave. He's clearly hurt, saying his feelings are "abandonados como tú en el viaje" [abandoned like you on the journey]. He remembers a time when "todo era perfecto" [everything was perfect] when they were together, but now he feels betrayed.
He's done with the games, telling her directly, "yo ya no te quiero, te amo, riesgo y lárgate de aquí" [I don't want you anymore, I love you, risk and get out of here]. It's a blunt dismissal, showing he's ready to move on. He even thanks God that his "corazón era de vientos" [heart was of winds], implying it can't be held down forever, and that this pain won't last. He's clear that he wouldn't have done what she did, and now, a part of him is "muriendo" [dying] because of it.