I am aware of that.
Again the true chance of autokill depends largely on your enemy. If you faced a cake slime for example, the autokill chance would indeed be roughly 50%. However, its not exactly that either. It would be 49%. If you were to face a powerful monsters, like a red wyrm, then the chance would drop to pretty much 0%.
This was shown in AO1, where it was described that although Elric's sword had a chance to stone, it had a 0% chance to stone when the underlord was at full health. Even at roughly 10% of his health, the chances were one in a million. Same thing with autokills. If a samurai chose to fight someone else at roughly the same power, the chance of autokill would be insignificant.