It may be unlawful, and it may fail because lower level officers refuse. But it seems like in that case the president cannot be charged. Lower level officers that go along with it could, but the president has the ability to pardon them.

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.