The supreme Court is specifically saying the order is legal. He could say it’s part of his official duties, in which case the order itself would be legal. His official duties include commanding the armed forces. If the president gives an order, a marine or a Navy SEAL cannot choose to not follow that order on legal grounds. They can choose to not follow on moral grounds but that refusal in itself would be illegal. Should it come to that, I would hope the vast majority of the armed forces would refuse the order.
In her dissent, justice Sotomayor specifically said that the president could order an assassination and could not be prosecuted for it. I am assuming she knows more than you are I about how the legal system works.
New England would like to come with you!