You can see shades this of this, e.g., in the difference between single-round versus iterated Prisoner's Dilemmas.
> MAD is utter bunk. It depends on rational actors that also believes the other actors are rational. Even Reagan realised the folly of MAD after Able Archer in 1983, and realising the Soviet leadership genuinely seemed to believe the US might be prepared to strike first.
What do you think the long-term prospects are of a species that goes around flinging RKVs at people? No more North America and no more Eurasia, if that happened. Maybe South America and Africa can pick up the pieces. Just because irrational hyperaggressive actors can briefly exist doesn't mean you're likely to encounter them. They won't survive for long.
MAD exists whether or not any particular participant believes in it, because it's just the cause and effect of competing powers each with their own agency. Or else we wouldn't be here. Even the Soviets knew that a possible US first strike was better than a guaranteed US retaliatory strike, which would happen if they struck first.
> Cooperation might eventually win out, but that won't help you if your civilization has long since been wiped out.
The whole "Fermi Paradox" arguments are based on an extreme form of "eventually, we should expect to see aliens, so why haven't we already?" This doesn't mean aggressive civilizations don't exist, but the reasons to think they're prevalent are overblown.