"I am a sick man… I am a spiteful man. I am an unattractive man." These are the opening words of Notes from the Underground, and indeed the Underground Man lives in a constant state of misdirected and paralyzing spite. His plight, he explains, is worsened by the fact that an intelligent man of consciousness cannot alleviate his spite through revenge; he is so conscious and so intelligent that he raises doubt after doubt until he is paralyzed in inaction. So instead, the Underground Man harbors his spite underground – even for the most trivial offenses – for decades.
The Underground Man hates himself more than he hates other men. His isolation underground, then, has less to do with an aversion to humanity and more to do with his attempt to take revenge on his own character.