I think you are giving drugs too much power. Sure drugs can lower your inhibitions, but it seems that DM was a quite uninhibited guy...racing, parachuting, multiple relationships with women at one time...
I think what we are seeing is simply DM's character in action.
As for MS, he has been quite inhibited in court, and even if he was a wake-and-bake type (no ashtray in the bedroom though) it would just make him demotivated. Pot is a peaceful drug: it is not like alcohol where goons take their bank card and health card to a bar looking for a fight.
I don't think this crime was committed under the influence of drugs. I think the influence was DM's character.