Author, there are many theories that Teach has two hearts, killing him for molesting Robin, just once and leaving him alive to start the war, and taking an arm off him, as a warning. You can make Teach tell the MC, if he has Robin, the world government is going to go after his head, since Robin is the enemy of the Government, and with this take the opportunity to tell everyone who is present, that Robin is his Nakama, if the world government wants her, then the MC declares a war on the government, no one can touch her. With this you prevent Robin from escaping from her, or think that they could betray him, since the MC declared war on the government for her, make it appear in the news. On the day of the war the MC is already in conflict with the world government, one more problem does not matter. It will be more fun since, the MC will be free to fight without any fear or consequences, since they are sworn enemies.