Mortal sin kills the life of grace, and venial sin inflicts a wound on the life of grace. In the case of a mortal sin, there are three necessary requirements for a person to have full guilt and culpability:
1. Grave matter
2. Full knowledge
3. Deliberate consent
If one, two, or all three are missing, then there is only the guilt of venial sin. Grave matter means that the act itself must be serious. Throwing a spitball may not be nice or polite, but it is not a grave matter. Stealing a valuable object or a large amount of money, committing murder, committing adultery or fornication, and committing perjury, for example, are all considered acts of grave matter.
Full knowledge means the person must know before and while he is committing the act that it is objectively wrong and sinful. Deliberate consent means that the person freely committed the sin. They were not drunk or stoned; they were not having a psychotic episode or hallucination, either. Being under the influence of drugs or alcohol impairs not only your judgment, but can also diminish your culpability, since you need to be fully conscious and sober to make moral decisions.
However, freely and deliberately getting under the influence is immoral and sinful. A drunk may not fully know what he is doing, but when he was sober and chose to get drunk or have those drinks, he made a moral decision to risk having his judgment impaired. Everyone is responsible for their actions, but especially when we have the consciousness and sobriety to know better. Driving under the influence of drugs or alcohol is a mortal sin at the moment the person decides (while still sober) to get high or intoxicated and makes no provision for a designated driver or for a cab ride home. The full culpability occurs when the sober person chooses to take a chance to drive later when he or she should not get behind the wheel.
Ignorance, force, fear, and habit do not destroy freedom, but they can greatly weaken or eliminate culpability. For example, I may be under the erroneous notion that you are a criminal and a thief. Instead of reporting you to the police, I take matters into my own hands and rough you up so you will admit your guilt. My ignorance of the truth (that you are innocent) and my fear may be so strong as to vitiate my freedom and culpability. It is no carte blanche, but it keeps things in perspective.