I have tried for hours to create an action that makes an actor into an aggressive enemy when a certain dialog is chosen. Any ideas?

StartCombat()?

I don't think it would make the NPC a permanent enemy (meaning they might accept a yield), but it should make them start combat with the player. Perhaps a combination of 'SetFactionRank()' and this would do the trick?

Something like...

Actor MyActor = akSpeakerMyActor.SetFactionRank(MyFaction,0 )MyActor.Startcombat( Game.GetPlayer() )

where 'MyFaction' is set to hate the PC?