"Mutual Assured Destruction" isn't a policy of anyone. It's a game theoretical idea that requires pre-conditions that don't necessarily apply. I mean, you hit on one of them: insane people. Most of the large nuclear powers have checks on that: the guy at the top can order the launch, but there's a bunch of relaying of the order. So the guys who turn the keys can refuse. So you need a bunch of insane people to make it happen.

And... also the Soviets once upon a time had a plan to nuke everyone, not just the US, in order to make sure that you don't have countries that build back up faster than they did. There weren't going to be any other countries that weren't radioactive.