My take on the gemini crew is this; They were not serial killers. As others alluded to, they eliminated threats. That being said, 99% of mob guys would kill as a last resort whereas the DeMeo crew would kill as a first resort. When you kill one or two people, thats usually tough to deal with, even for a mob guy. When you kill over a certain amount, you become immune to it and killing just becomes the best solution. I believe thats what happened to the DeMeo crew. They lost all sense of compassion.