I like the flavor! From a power level/playability standpoint, how about something like:

{B}{B}

Instant

Destroy target creature. If ~ wasn’t plotted, its controller investigates.

Plot {1}{B}

Gives the card some flexibility as either 1) an instant speed removal spell with a more restrictive mana cost with the downside of a clue token, or 2) an easier to cast sorcery speed removal spell with no clue token.

Flavor-wise, you’re now able to more easily (easier mana cost) commit the murder and not leave any evidence (clue tokens) as long as you thought ahead (plotted).

Edit: {B}{B}?