I didn't claim anything about the "inherent peaceful nature" of the USSR. It was an empire; empires are generally not peaceful.
However, there's a very simple and obvious reason as to why this mode of thinking prevailed after Stalin's death. The leaders who subsequently took power were those who saw the devastation of WW2 very close up, and knew full well of the true economic and social cost (to remind, Soviet losses were 20 million officially, but the real numbers were quite a bit higher).
The result was a firm commitment to "never again!" - that is, regardless of ideology and politics of the day, the axiom was that there shouldn't be another devastating war like that in the Russian heartland. And it was obvious that any major war with NATO carried a significant risk of such devastation - Soviets simply didn't have enough resources to guarantee the upper hand in a conventional engagement against the combined forces of US and Europe.
(Stalin didn't care about any of this - to his mode of thinking, sacrificing 20 million to grow his power was perfectly acceptable. That's why, as you correctly noted, the change didn't happen until some time after his death, once his political legacy waned.)
Note also that this explains some other things, such as the occupation of eastern Europe - its primary purpose was to serve as a buffer zone in any conflict with NATO, so that any territorial losses and devastation accompanied by large-scale warfare would be concentrated in those areas, as opposed to Russia proper.