Should Deathstroke turn foe at a critical moment in the battle, we could see lots of casualties in the Arrow world.
Now free, Deathstroke will not only get a chance at real escape, but also a chance to further exact his revenge on Oliver should he choose to do so.
Oliver had a hard time taking on the suited up Deathstroke their first go around, I can’t imagine what would happen should he flip the script and align with Prometheus.
One would assume that Slade Wilson fulfilled a chunk of that promise once he executed Moira Queen, but both Thea and Oliver took the death of their mother fairly well.