One hit kills? No.
Scenarios should vary. The bad guy role player is instructed to react after so many shots fired or after so many hits. The purpose is not to test marksmanship against a live target, but to test tactical decision-making under stress, i.e., does the good guy react appropriately to the situation, does the good guy sense threat cues, does the good guy underreact or overreact?
There should also be scenarios where, if the good guy achieves a position of advantage, no shots are necessary - just verbal commands at gunpoint.
And there should be ambiguous scenarios - in which the bad guy doesn't react in a manner that the good guy expects.
Some examples of scenarios:
Solicitor at door (casing for burglary turns violent)
Meter reader (casing for burglary turns violent)
Home alone ill (burglary in progress turns violent)
Return home, interrupt burglary in progress
Intentionally targeted
Someone with a grudge
Mistakenly targeted (mistaken ID)
Car prowl outside house
Suspicious activity outside house
Vandalism outside house
Jilted lover/admirer/stalker
Home Invasion
Burglary
Psychotic/irrational person
Person on drugs/drunk
Police hot pursuit bad guy enters home
Unrelated criminal activity spills onto property/into house
Activities the Good Guy could be doing when the Bad Guy appears:
Laundry
Washing dishes
House cleaning/straightening up
Watching TV
Using PC
Studying
Bathing/personal hygiene
Getting ready for work
Cooking
Sleeping
Talking on telephone
Washing car
Mowing lawn/gardening
Just some outside the box stuff to think about.