Im posting this here because im not sure if theres somewhere specific in SOT game discussion to put it.
My current theory as to who killed De Marco is not any specific person, but the grand maritime union! The way i have come to this conclusion, is because the pirate lord him self, who seemed to come to the sea of thieves to escape the GMU, suddenly seems to be accepting them into the sea, and allowing their ambassadors at the outposts... Why would some one who is so against the GMU suddenly embrace them? My thoughts are that they killed his son, and threatened to kill Lesedi if he didnt comply with them.
So hes stuck with a choice of either allowing them to kill his only living child but keep the sea of thieves free of their influence, or keep his daughter safe, but allow them free access to his domain. If this is so, i hope that eventually he makes the right choice and defends the sea of thieves from the reign of the GMU, perhaps he will find a way to keep both his daughter and the sea safe from them.