On the one hand, I’d say this: if you did not have full knowledge that the person had a real gun and that he intended to use it unjustly, then shooting him would be immoral. However, if you had the flu, theoretically you wouldn’t be thinking clearly, and be unable to control your vague, semi-rational impulses, and thus, if you do shoot him, you would not be culpable at all potentially … in which case, it would not be immoral, and not be an issue of morality at all. That is because, for morality to apply to an action, the agent (i.e. the doer of the action) must be in control of himself. It’s a basic rule of gun safety … KNOW YOUR TARGET!!! Don’t shoot at something rustling in the bushes. You can be put in jail if you said, “Oh, but I was pretty sure it was deer,” while it was actually a person. That’s no excuse, both in natural and human law (at least in human law in the U.S. currently … ).
On the other hand: what if there was an especially deceptive situation … like if you happened to see a section of a film set that you thought was real life and you saw someone [an actor] about to shoot another person [with a blank, unbeknownst to you]. It’s been made to look perfectly real … so would you shoot him then?
What comes into play here is studied ignorance vs. invincible ignorance. Studied ignorance is where you “should have known better,” in other words, you willfully were not diligent enough to make sure you understood everything in the situation as far as you were able to. Your ignorance would, in some way, be called sinful, as it could have been prevented by your will. Invincible ignorance is where you did do everything in your power to learn about the situation in question but you, through no fault of your own, still did not know an important detail … leading to, what would normally be an immoral action, but in this case an inculpable action … and not an immoral action either. Still, you made a mistake … an innocent one … you made an error … and yet not a sin. And yet, theoretically, if you truly were completely innocent of any neglect whatsoever … would you still have shot the innocent person despite deceptive appearances? Could the Virgin Mary, even, innocent of sin, and yet not omniscient, make that mistake? You know, I have no idea. I invoke a more learned person’s answer to rescue us from this predicament.