It is the point over which the dissent occurred that is of interest to you and me.
The relevant offence was double-barrelled: committing a drug dealing offence, while in possession of a firearm. Does the secondary party, who assists or encourages the offence, have to help with both the dealing and the firearm possession? No disagreement here: it is only necessary that the defendant provides assistance or encouragement with some part of the offence. Does the secondary party have to know of all the circumstances – both the dealing and the firearm? Again, no disagreement: the defendant must know of all the circumstances of the offending.
But what if the defendant, present during the commission of the drug dealing, only becomes aware of the firearm after it is too late to withdraw from participation? The defendant may not be able realistically to say, “Everyone stop! No guns!” because that might risk the safety of other people or even of the defendant himself.
Here the dissenters say this is like the affirmative defences of duress or necessity: the defendant is saying he can’t withdraw because of forces beyond his control. He should, according to the relevant law (not universally applicable), have to prove those defences. They do not negate an element of the offence.
The majority disagreed with that (slip op, p 10 footnote 10). The question is one of fact: what did the defendant intend to assist or encourage? Evidence that he only learn’t of the firearm after the drug deal had commenced would be relevant to whether he intended to assist or encourage the firearm possession. His continuation with his participation could be consistent with him encouraging the gun element, or not: that is a matter the fact-finder would have to determine (see slip op, p 13, footnote 9). The prosecutor must, when such issues are raised, prove the necessary intent without a burden of proof passing to the defendant.
Defences like duress or necessity could, on relevant facts and subject to local law, be available, and again, subject to local law, may only require the defendant to raise them as live issues by pointing to some supporting evidence.
Normally, aside from in offences that include a proscribed purpose, the law does not concern itself with the motive behind an intention; Robin Hood was a thief. Where an intention existed because the defendant faced circumstances that it would be beyond normal human fortitude to endure, a defence of necessity might be available. But not necessarily. This is where policy limits on justifications and excuses operate. In Rosemond the Supreme Court majority lowered the moral hurdle for defendants by allowing a motive for an intention to limit responsibility even though the stricter requirements of an affirmative defence would not be satisfied.
I doubt that this would be a universally acceptable recognition of human frailty.