Among the possibilities: they could have targeted Monkey, Kraken, or perhaps Ment was Norrington and framed him for the murder.
They could only have targeted Monkey if Ples is indeed Priklop; otherwise, it would have been extremely foolish of them to attack him. Overall, yes, this idea is also plausible. So, either the Mafia killed Ment, or they ran into someone with natural protection, or they targeted Jack, and he sent the assassin to Ment/someone with natural protection/himself.
Master, doesn't natural protection show up in any way when it triggers?

