I personally would not take that shot, I would confront the guy though, if he then presented a threat against my life or the life of others I would fire. It is easy to say that you could kill someone in a hypothetical situation but in reality it is alot harder then that (outside of psychopathy). Especially if the reasons for doing so are foggy at best. Hell, some folks still end up with PTSD or insomnia even when they took a shot in obvious self defense.
Also, Ethics is like nutrition. It has barely anything to tell you about these lifeboat situations. Its the same as going to a doctor when your 70 years old with 150lbs of fat, throat cancer from a history of chain smoking going back 30 years, a liver thats about to collapse from overdrinking, a limb thats falling off, and a brain tumor and then asking the doctor to fix you. Hes pretty much just gonna tell you your screwed.