Some church documents refer to circumstances that reduce culpability for a mortal sin, which seems to imply that it does not eliminate culpability. What does that mean?
We are more or less guilty depending on 1) how serious the sin is in itself; 2) our knowledge that we are sinning; 3) our *free *consent to sin anyway. If grave matter, full knowledge or full consent is lacking, the sin is not mortal.
Factors like ignorance, addiction, mental illness, and coercion can make our acts less free and deliberate, and therefore less serious, subjectively speaking, although they may be serious in themselves. Note, however, just because one of those factors is in play, doesn’t *necessarily *mean the sin is not mortal. And just because it’s not mortal, doesn’t mean it’s no big deal. A good confessor can help you discern in a given case.
It is all about God’s everlasting love. He have given His Church the power to reduce some sins in some circumstances so that we all can be saved. It is God’s love and mercy, and we should each and every day think about that. After all evil we have commited, God still loves us. And all He want us to do is His will, teached by His Holy Catholic Church.
This is a complex area, but certainly one can commit objective sin (something that is inherently wrong) but have no culpability for the sin (no personal sin committed). Unintentional ignorance - not knowing something is a sin through no fault of your own - can completely remove culpability.
So yes, it is possible that one can have no culpability whatsoever for a given grave sin committed.
In general, though, when the Church talks of reducing culpability it is a recognition that circumstances can affect our decision-making so that we are not completely free in our choices. These circumstances do not completely rob us of free will, so we retain some level of fault for our wrongdoing.