So despite all the critism i have gave to Eobard95 for thinking villains turning good is a bad idea. The idea shown in Arrow's sizzle reel actually looks like they are executing the idea right. See Deathstroke seems to be joining the team because of the fact that Oliver's team is slowly being taken out by Prometheus. Makes sense, however i'm thinking Slade will not be entirely turning over a new leaf. He'll probably take advantage of Oliver's desperate nature. Plus i think one thing in it for Slade is to finally do the one thing he said he would do.
"I always keep my promises!"
He still hasn't done it. However there are those in Oliver's life that he cares about the most. We are not sure who will die in the end. It is an Arrow tradition though so it will happen.