Nicky Scarfo and paul Castellano each made a bad decision to whack one of their top guys. Scarfo whacked Salvie testa out of pure jealousy which made other Scarfo soldiers eventually flip since he killed a guy who was well respected. You can say a lot of things about Paul castellano not being a true gangster but he still was a smart guy who made his family successful. However, one could argue that if Castellano never ordered Demeo killed he probably would have never been whacked by gotti since gotti feared the demeo crew.