Full-blown psychopaths aren't really functional, and could hardly be expected to run a racket.

Perosnally, I do not think DeMeo suffered from any mental illness. He just liked to kill people, but that doesn't make him insane, just means he got to do what the vast majority of people never get to do, that's all.

Same for the Twins. Although their coke habit certainly would have reduced their inhibitions.

Neither Roy DeMeo or the Gemini Twins would classified as insane under the legal definition.