Learn Spanish (Latin America): No me molesta en absoluto.

The subject is implied, usually referring to "it" or "that" (eso). Molestar works like the verb gustar: the thing causing the bother is the grammatical subject, and the person being bothered is the indirect object (me). Literally, this structure translates to "It does not cause bother to me."

Why do we use en absoluto here, and could I just use nada?

Yes, you could easily say No me molesta para nada or No me molesta nada. En absoluto is just a slightly stronger or more formal way to say "at all" or "in the slightest." It adds intense focus to the negation, emphasizing that the level of bother is exactly zero.

If I drop the No at the beginning, can I just say Me molesta en absoluto?

No, because en absoluto requires a negative word like no earlier in the sentence to complete the meaning "not at all." However, if someone asks you "¿Te molesta?" (Does it bother you?), you can reply with the standalone phrase En absoluto, which means "Absolutely not."

How does en absoluto compare to a focus adverb like ni siquiera (not even) in this context?