An antihero is "a protagonist who has the opposite of most of the traditional attributes of a hero. He or she may be bewildered, ineffectual, deluded, or merely pathetic. Often what antiheroes learn, if they learn anything at all, is that the world isolates them in an existence devoid of God and absolute values."  The Misfit qualifies as an antihero because he possesses stereotypical hero qualities yet he is a cold blooded killer.  He is respectful - he asks politely for each of the family members to step into the woods, helpful - he stops and initially seems to only want to help with the car, gentlemanly - he apologizes for not wearing a shirt in the presence of ladies, and he is the only character the reader can sympathize with as he shares about his past, his family, and his struggles.  Yet, despite all these qualities, he is deluded - he quietly and systematically kills the entire family.  He agrees with the grandmother that he has come from good people and appreciates her compliments, yet shoots her at point blank range.  

In addition, as the definition above states, the Misfit blames society for isolating him - he feels he was sent to prison unfairly, and he blames Jesus for throwing "everything off balance".  In the end, despite a hero facade, the Misfit is a deranged murderer who concludes the story by stating their is no real pleasure in life.