Deathstroke Helps Lex Luthor Escape

Speaking of credits scenes, we learn that Lex Luthor has escaped Arkham and is pursuing his own nefarious plans, with some assistance from Deathstroke.

The scene works on its own, but details are missing. For example, given Batman's connection, how does Luthor manage to escape? Has Luthor already met Deathstroke, or is Deathstroke just a gun for hire?

In a deleted Justice League scene, Slade Wilson helps Luthor break out of Arkham. They part ways, but not before agreeing to meet at a previously designated place. So this means that the two have been in contact for some time.

Just another essential scene that didn't make it into the film.