For Barlow and Kubecka their murders made too much heat on the family. If they had tried to solve the question in silence would be better.
Yet, unfortunately these specific murders went unpunished (almost): the triggermen were never prosecuted, Salvatore Avellino who ordered the murders got 10 years only, and to Casso who authorized it the outcome didn't make difference anyway, because he got life sentences for other murders.
That the triggermen didn't get even 1 year in prison is a shame. I remember some movie where the main character (a policeman) says about the gangster: "He will pay for what he has done even if I have to make up the evidence with my own hands!". Maybe the real-life cops should have done the same in this case imo.