Odd, you just stated as to my hypothetical situation that the sin was murder, committed morally. In other words, the intent to murder was formed and willed. That is what made the act culpable.

If you attack me with a knife, and I have a gun, and I point it at center body mass and pull the trigger, my intent is to stop the attack and my will is directed to that end. If you die, from a moral standpoint, it is incidental.

We cannot be held morally culpable for an unwilled act.