What you describe is duress and yes, duress *can *reduce the culpability of the sinner.
That said, we cannot presume either way here. With a gun to my head, I may be fearful and feel duress. But equally, if I am able to comprehend the choice before me - kill or be killed - I may still have sufficient freedom to make that choice and be responsible for it.
If the duress limits my free thinking so that I instinctively just pull the trigger to save my own life, then perhaps I lack the deliberate consent to make it a personal mortal sin.
However, if I contemplate my precarious situation and decide that I’d rather kill than be killed, and then choose to pull the trigger, I have made a choice and am responsible for the mortal sin I commit.