From what I understand, murder is illegal in the civilized parts of Westeros and Essos. If it weren't, I would expect more people to die on the streets than they seem to.
The Faceless Men are hired assassins. If they are paid to kill a king, they will kill the king. If they are paid to kill a noble lord, they will kill the noble lord.
How aren't the Faceless Men put on trial for their crimes and essentially destroyed?
Even if they have religious cause to kill people, murder is still illegal.
Also, if I were a king of Westeros or a noble man in Braavos, I would not want this cult to survive because while they exist, one of my enemies could have me assassinated.
How have the Faceless Men survived?