Question about Russell's instructions on handling "Doc"

He tells the "kid" to hold a gun on Doc, and says something like:

"If he wants to leave before they come back (meaning the bandits, I guess?) okay, but not after."

Then he goes on to say to put one bullet in him if he leaves doing one thing, two if another (I don't remember what those were), and finally "if he tries to take the water, fill him full of holes" or something along those lines.

In particular, I'm wondering why he was saying Doc could leave without being shot if it was before "they come back"? Or did I just understand the whole thing wrong?

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