Not sure about the instant kill roll, but it shouldn't be a problem: Assuming 18-20 is a crit, and 14-20 confirms, and 14-20 leads to instant kill, the probability that this happens is: (3/20)*(7/20)*(7/20) = 0.018375, or a little less than 2% of the time. (Note: This is the player's best case...