You can't understand them because they are not like you or I, they are different. You know that part of you that can know what others may be feeling or it sort of keeps you in check. They don't have it. It's like something is missing from them.
Once I flew into rage at someone, the rage took over and I had the person by the throat and was squeezing, I was doing this without knowledge, as I was squeezing their throat I sort of came to and realised what I was doing and freaked out letting go, the person was in shock and fear, I shall never forget it. Now, a Psycho doesn't have that part that kicked in and stopped me, they just keep going. Where you of I would feel guilt, remorse etc, they feel nothing.
Narcissism,is based on a Greek Myth I believe.
A little boy went into a Forrest and he came across a pond, he looked into the pond and saw his own reflection in the water, he had never seen his face before and so when he saw his image for the first time he fell in love with it, he fell in love with his own image and sat at the edge of the pond pining and longing and died there. Where he died a flower sprung up, the flower was called a Narcissus.
So Narcissism is a very distorted and unhealthy self love. To the Narcissist other people literally don't exist. It's hard to imagine but a person like this can only see themselves, every body else is an extension of themselves. They do not understand that other's exist and have choices, feelings motivations, that can be different from themselves.
A great wound lies at the centre of Narcissism, they come from very unhealthy childhoods. Whether they are born like this I don't know.
I hope this gives you a little more insight into the whole thing.