There's a relatively small minority of the world population who live in one of the two countries with arms sufficient to "destroy the world" (although some of the runners up could certainly make life worldwide more miserable).
In practice people living in those countries have pretty much all the say in whether the world will get destroyed or not by choosing to vote for some nutcase in the next election. By comparison someone in <name pretty much any other country> has next to zero chance of moving the needle on potential world destruction.
Nationality plays a role in who has the actual power but anyone can promote peace and have a real impact. Influential words and good ideas can come from anywhere.
But the main moral culpability is just not caring. Nuclear safety isn't even on the mind of the average voter in the US or Russia. If it were an issue as important as major wedge issues as say guns or religion are in the US even a small fraction of the population could have a major impact.