I don't think this is the case. All military members of all branches are taught that they must not obey unlawful orders. Killing elected officials is clearly unlawful.
And the president is still subject to impeachment.
And the president is still subject to impeachment.
So even if I some case they fail, there are no consequences so people can try again.
Yes there is also political considerations, but if the president feels confident enough to plan assainations of a rival, or for instance a coup with others in the executive branch, then he must feel he has that handled.