Question: In the RtL campaign, when a hero kills a monster during its activation with a guard attack and the monster succeeds the undying roll, does the monster resume its turn as normal, or is the monsters activation over?

I don't believe the Undying ability says anything about immediately ending the figure's turn. It triggers as soon as the figure reaches 0 HP and its effect is to restore full HP and wipe off lingering effects. Yes, the undying figure could continue its turn exactly where it left off.

I had a feeling it went that way, but thanks for the reply.