The General Rules.
Only non-hostile NPCs and Bartenders count. So no hostile Lawmen, Sheriffs, Deputies, Guards, Strangers, ect at their time of death.
Any use of Express Ammo or such should be noted in the title.
Doors can be blocked.
Times are approximate.
Times begin during the second of the first shot and ends about a second after the last confirmed kill.
Kills must be confirmed by Honor Points lost.
No Deadeye.
Must not stray from exits or map location.
Must enable Slow Motion Killcams.

Misc.
The time is more for fun and the time isn't exact.