He described himself a "violently anti-communist", and he saw nuclear war as an eventual necessity in a civilisational conflict between democracy and totalitarianism. If you view a nuclear war as unavoidable, it's easy to conclude that it's better to get it over with sooner rather than later, since a later war will be even more destructive. Especially since von Neumann was active in the 1940s and 50s, when the west had a slight edge in weapons technology, and before the development of the giant H-bombs. He and a few other pro-nuclear scientists argued there was a brief window where the USA could fight and win a nuclear war against the USSR, and they should take it.

While it's easy to dismiss that view in hindsight, it's worth considering that von Neumann had personally witnessed the rise of the Nazis and had to flee along with many of his friends and family. And he had seen that regime defeated only by military strength of the USA. And while the USSR and USA never did end up in a nuclear war, it really was only by a series of miracles that it was avoided. I think most historians of the 20th century would agree we got very, very lucky.