Dead men indeed tell no tales. However, the Penntons were delusional to even conceive the idea of killing Afriana Hastings. A small and inexperienced family like theirs is not enough to bring down the underworld's most fearsome woman.
Iron lady cannot fall at the hands of the Penntons, however, the destruction they invite into their house would surely consume them whole. That would be in the not-so-long future, as Adriana would allow them to enjoy the short victory they think they earned through meticulous planning. It is absolutely a disservice to burst a person's bubble, and that too, when it's so soon.
Robert Pennton immediately began putting things in place for his daughter's future. Actually, he left out one last angle that they would need to turn their arguments too. Robert plans to declare his daughter as a mentally unstable person. The attempt on her own life would be one example to cite.