I could tell that the arm is there... Where you see that curve of the cape, that's the forearm's location. So, it's not so "odd" if you ask me. What would be weird is that the cape was straight all the way, but its not. Plus, Battercup (Buttercup in Bat-suit) is in a defensive position. I'm sure when Bleedman drew it, he had hinted the location of the right arm position.
@ Tom E. Gemcity - Are you talking about Arkham Origins? Deathstroke was pretty easy to beat - And yes, I had the game in Normal Mode. Quite an epic entrance and battle!