The sneak peek indicates that he also seems to believe that the mask just washed up on shore after the sinking of the Amazo and propped itself up on the piece of driftwood; something tells me that we’ll learn that it wasn’t quite an act of nature that left the mask in that position to be arrowed by Oliver.
Apparently, Slade Wilson’s Deathstroke mask came to be impaled with an arrow and left on display by none other than Oliver Queen when he and Anatoly returned to Lian Yu after their Bratva adventure.
As this was before he discovered that Slade was very much alive, Oliver was understandably sad to see the mask of the man who had been a very good friend, prior to the whole “go crazy and attempt to murder” twist.