I suspect Deathstroke will appear again. We did not see him die in the flashbacks. We know we will see more of those. When Oliver was found Deathstroke's mask with an arrow through the eye hole was shown.
It appears they are setting him up for more. I would hope he is still alive in the present for a full episode about him. I do think the show is using comic characters to lure fans but not using them very well. Hopefully Deathsroke will appear again. But they used the publicity pictures and trailer to make it seem he was a huge part of the episode. Similar with Deadshot. I was not impressed with him. Other than the eye piece, that could have been any generic assassin who was quickly killed off.