@astrotheological,
Ok, you must realize that killing is wrong. Just because somebody tells you to doesn't make it right. And any alien encounter should be automatically viewed as a hallucination, not reality. So distinguishing between the two shouldn't be hard with something so far fetched like that.
So the person would probably go into a mental institution if he elaborated on his situation, because trusting a non-existent being is just wrong.
Besides, how is it any different to the dead person if one hallucinates or is in a perfectly healthy reality.