Why, he would murder them, though it goes with the job.
Actually Harry is higher ranking than Seven and he has ordered her around before, see Nightingale.
It would be Paris or Janeway.
If it's consensual and non-criminal, with Starfleet members accepting this risk at enlistment, I don't think you can defend the use of the term "murder" for giving such an order.
Seven doesn't have a rank as far as I know, so I am not sure how Harry can outrank her as such. Her voluntary participation in operations, which she withdraws whenever she gets pissed off or has a different opinion, cannot be equated to rank, or even to whatever binding agreements I am sure civilian scientists and whatnot who do tours on Starfleet ships have to sign.
In what situation would Kim be ordering Janeway around? Can't think of any way that could take place in the Voyager scenario.