If neither of us are lawyers, the next question is to ask which jurisdiction are we not lawyers in?
Because current UK law requires that
intent be proved as well as foresight. The case I quoted from, R. v Jogee, was the case that established that.
Your argument for the prosecution is that Zilwicki and Cachat might
reasonably have known that David would run off and nuke a park. Complete with kids.
The argument for the defence would be that Zilwicki and Cachat did not encourage him to do that, did not plan to do that, and in fact asked Carl to tell him not to blow the nuke at all. They are in the position of a group of people off to burgle a house, who tell David to leave the stolen gun in the car - only to have him take it with him and shoot the householder.
In UK law, the provable lack of 'intent' of the others would mean they were not accessories to the murder. Even under the old common purpose cases, they'd have probably got off, because the defence could argue that they couldn't
reasonably know David wouldn't do as he was told.
Now, my experience of criminal cases (no, I'm not a lawyer) is that it would be bloody difficult to convict Zilwicki and Cachat. Yes, you could argue that they knew he had a nuke, therefore might reasonably expect that he could use it. But as well as the question of whether they could
reasonably know David would decide to disobey the rest of the group, David had definitely broken from the 'common purpose' of the group.
People keep quoting the getaway driver - at this point Zilwicki and Cachat (and Carl and Karen and the others) aren't getaway drivers. They're the gang who've just robbed the bank, are driving away - and watch the unstable member NOT get in the getaway car, but instead take the gun he was supposed to use in the robbery and rush over to the local park to start shooting.