The command card "Lord of the Sith" for Darth Vader reads as follows: "Use during your activation. For each hostile figure defeated during this activation, you may perform 1 additional action".

Am I right in understanding that since the card says use during your activation instead of use at the beginning of your activation, I can wait to see if I kill a hostile figure say with an attack before playing the card? And if that is true, I can still perform an additional action as the hostile figure was defeated during this activation even though it was before I played "Lord of the Sith"?

Thanks for the clarification!

Yes

So a lucky Vader who has the perfect set-up could potentially get 3 additional actions? (take out 2 with brutality and finish off a badly wounded 3rd with force choke)

But can't use them to make more than 1 attack and 1 special action...

And then Telekinetic Throw.

And then Force Lightning.

And that's exactly how Vader puts the "F U" into "Force User".

True story.

Using Lord of the Sith to get off Brutality, Force Choke, Force Lightning, and Lure of the Dark side all in one activation is one of the most satisfying experience in Imperial Assault and is the reason why playing a Vader list is so enjoyable.