There were many reasons Orion didn't kill him just now.
One of them was that he and Mark were in similar situations. Having multiple elemental talents made Mark a versatile fighter, and Orion knew exactly how dangerous someone with control over many elements could be.
To counter Mark, Orion would need to use his full strength and his arsenal of curses. But if he failed to kill Mark, or if Mark somehow escaped, things would get much worse.
Mark already knew that someone was impersonating Ray. After seeing his different elements and strength during the fight, Mark would come to a shocking conclusion—Orion Darkwood was impersonating Ray. If that happened, Orion's situation would deteriorate.