A character with Aggressive Negotiations will lose Diplomat if they are targeted or attacked, or if they attack or deal damage. So if a character with AN deals damage with Turn To The Dark Side, they will lose Diplomat. This isn't clear from the card or glossary text- which currently doesn't say that the character loses Diplomat any time you deal damage- because the ability received an errata.
Quote:Aggressive negotiations- the ability will be amended so it’s switched off whenever a figure causes
damage to an enemy (“…until it makes an attack, –deals damage to or– is targeted by an enemy
character”).