I think JvN deserves at least some benefit of the doubt. As far as I am aware there is little unbiased impartial information about these meetings discussing the targets for nuclear strikes.
Most information was published well after JvN has died. He also had no chance to comment even in the last years of his life. (Apparently the military was near him in hospital to make sure he didn't reveal any secrets)
e.g. JvN's recommendation to bomb Kyoto is usually supported with a quote from a book that was written by the general who ran the Manhattan project and published his account of the events five years after von Neumann's death. I wouldn't be surprised if the general was doing at least some white washing in the early sixties as the peace movement was gaining momentum.
More recently, looking at how various US agencies and departments reacted to accusations of human rights abuses in say Iraq, it's very easy to imagine that JvN's comments on the topic where either ripped out of context or exaggerated.
So again, while it's very clear how someone with an extremely scientific mindset like von Neumann's would through outrageous comments around during a brainstorming session or during a heated argument. I'd really try to withhold any judgment on this until I see the full transcripts of these meetings.