good reason never to hold anyone at gun point, or to shoot anyone who, upon being told to halt, gets within 10 feet of you, 21 feet if you see a blade. Shoot early, shoot often.
One thing, however, if we assume we're doing this in an airplane, we're going to be channelized by the seats-"surprise" is going to be difficult to come by. i suspect it's going to take a "hi-diddle-diddle, right up the middle" approach. To whoever talked about getting 'em on the ground, yeah, makes sense- one guy plan on taking him down and absorbing the (almost certainly crippling and easily lethal) cuts, while the second guy disables the bad guy per above.