The whole point is that even if he would never press the button that's fair enough but he's supposed to keep stumm and say yep he'd do it. That's it. If any actually start flying then we are into a different pretty doomed scenario.
He didn't even need to say he would or wouldn't - and would have been fine to say he'd take a progressive policy and any situation under advisement (he could even hide behind interests of national security though that would be more a right wing style of politics) - but he took the first opportunity he could, against the interests of the country, to try and break the system and promote his own vision of an end goal of a nuclear free world - and people think voting for him as PM is a good idea...