My moneys on things happening another way or two.
1. Ollie kills the dark Archer in front of Tommy who finds out its his father and sees Ollie as the Arrow. Vowing revenge he takes up the mantle of the Dark Archer and vows to destroy Ollie & Laurel. I like this one because the young actor is cheaper to keep around than Barrowman.
2. After a long battle between GA & DA where Tommy and Laurel are held captive both Archers are revealed to the captives. After more fighting Ollie is nearly beaten but Tommy takes the kill shot. Ollie then "kills" the Dark archer. Laurel conflicted tells Ollie she never want to see him again. Which gives us our angst for next season.