We cannot say all narcissists are completely aware of why they are acting out. Narcissistic Personality Disorder exists on a spectrum and every NPD person will be different even though narcissists do share some common narcissistic traits.
It depends on where they are on the spectrum. Some do act out compulsively in defense of the false self. Most all narcissists even when acting out compulsively are able to look at you, see the pain on your face and know they have caused you pain. The fact is they don’t care. They think you deserved it. If they resort to physical abuse they most definitely know they are hitting you and they are very aware this is hurting you. Narcissists are driven to protect and defend the false self this is a defense mechanism. It is this defense mechanism which kicks in automatically when they feel threatened or injured. This part is unconscious. They are fighting for their life when they are protecting the false self. This false construct is all they have this is why they work so hard to protect it. The real self no longer exists it was killed off in childhood.
While it may be a compulsive act to defend the false self, they can look around and see the destruction. They are still responsible for any acts of violence which they commit. The courts do not recognize Narcissist Personality Disorder as a defense for those who commit acts of violence.
When the Narcissist is devaluing you they see your pain and this is fueling them. Depending where they are on the spectrum many will not know it is fuel or narcissistic supply which is driving them. They may only realize they feel much better after devaluing their partner. They are aware they are hurting you, but hurting you makes them feel better. The need for fuel or narcissistic supply is unconsciously driven. Aware Narcissist are extremely rare.
The narcissist is very wounded. He feels slighted by something someone says or does most every day. When he feels narcissisticly injured his defense mechanisms kicks in (unconsciously). He has to do something to recover from the wound or injury. He recovers by extracting narcissistic supply. Any thing which validates the narcissist false self is narcissistic supply. If he cannot find admiration he will extract narcissistic supply by devaluation. Any form of attention good or bad is narcissistic supply. This can go on all day everyday. Most narcissists are not consciously walking around saying I must find narcissistic supply or I must find fuel in order to recover. These actions are unconsciously driven. They are not saying to themselves I must destroy my spouse or partner in order to feel better.
They are aware they have resorted to abusing you, they may not fully know why. They only know they feel better after doing so. Their need to recover from feeling slighted or narcissisticly injured is instinctive. The fact is they will always put their need to feel better before you. If they must abuse you to feel better then so be it. The narcissist is not thinking about you, or what this abuse is doing to you. He is totally self absorbed. He can only think of himself and his own needs.
A narcissist is aware he is hitting you, or cursing you, and saying the most horrible things to you. He is aware this is causing you pain. What the narcissist may be unaware of is his own process and why he resorts to abuse. Having a personality disorder should never be used as an excuse to abuse others.