1) Total lack of ethics - and the resources to get away with murdering thousands of human test subjects along the way.
2) Suicidal tendencies - not just for individuals, but for the funding group. Because any realistically dangerous weapon will have a good probability of killing it's creators first, and a very high probability of killing it's creators in the long term (either directly, by evolution, or by revenge nuclear attacks.
3) You still need Highly intelligent and highly trained people involved. Most of whom lack Suicidal tendencies.
This armegeddon scenario is actually far more likely than a nuclear war, as that requires far more people to behave far more stupidly and unlike nanite fears, is actually physically reasonable.