I like the idea that the spy-pen will turn out to be Keith's salvation.
It seems the spy-pen is what made Keith realize he had to run back in, and he was shouting, "Kendall,"
My take was that when Keith found the spy-pen he realized that they had been followed and were in danger, so he went for the gun in the glove box. The gun wasn't there, and Keith realized that Cormac must have taken it while Keith was napping during the wait for the tow truck. Keith realized that Cormac took the gun because he wanted all the money, not just a share. Perhaps Keith didn't think that Cormac would kill him and Kendall -- he hadn't done so at the earliest opportunity, after all -- or perhaps he just didn't think beyond "Aaaaagh!" Kendall was still his client. Either way, he went running back to the house.
As to why Cormac didn't kill Keith earlier, perhaps Keith never told Cormac exactly where Kendall was hiding out, and Cormac waited to do any shooting until he had actual possession of the money bag.