Six-year-old johnny suddenly makes an angry face at alan and kicks him hard for no apparent reason. it is clear that he means to cause pain to alan and is not just "playing around." johnny is displaying:

Johnny is displaying antisocial behavior.
He is showing signs of aggression and unacceptable behaviors which are not appropriate within his society. He is hitting another child without any particular cause - he just wants to cause him harm and pain. This is a sign of antisocial behavior which can be a huge problem for Johnny if it continues into his adult life.