IMHO, it was DM the psychopath at work. It's pretty hard to take the guy you're trying to hang the whole thing on with you. There's absolutely no way that MS would freely take a gun if he knew that DM had killed TB with it. I doubt he'd want to take anything that belonged to DM and could incriminate him. IMO, that's why DM had to call in the services of another trusty mission helper- CN.
If the theory of a set-up/fall-guy scenario holds any water - i can't but help think that putting the alleged gun in the tool-box, and the alleged 1 lb of weed in a nap-sack in the hands of two separate people, was a deliberate measure done by DM. If one or the other, didn't succeed in making the entrusted evidence get into the possession of MS, then there was still one other major piece of evidence that would. Where that theoretical little plan would go south, is exactly how it went down .. both got together, ditched the evidence, and made a likely already troubled MS, absolutely paranoid given the circumstances of the abandonment of these two items in a common spot.
Not saying MS is innocent in this at all .. i do think that he was in the truck when it went down - I just don't think he was the shooter. I'm thinking post-crime, words were said between the two, that made DM decide that it'd be better for him if MS went down for the shooting - or anything else for that matter to get him out of the way, and tried to plan accordingly.
IMHO.