What Causes Antisocial Violent Behavior In A Child?

Good evening Dr, I have a 6 and half year old son, last week his female friend told him that he is not smart ,then he got mad he said back to her that he would get cut up by her father then he would cut up the father. the girl was crying hysterically. That is the whole note from my son teacher wrote to me. I came home and asked my son the reason he said that, then he told me the girl said that he is not smart then he got mad and said back and he told me that his saying was not real. Now I am so worried so much for him, he is a very active kid , and his idea is about his is the good one than other kids Last year when he was in K2 he was scaring other friend in his school when he ate his breakfast in lunch room he saw a plastic knife on the table then he took it and roll in a napkin and put in his pocket pan and went to the bathroom then showed to other kids around there and told them that he had a knife and he will cut off their heads, that was made other kid scare and run away. After that happen I remembered that lately before that he read some books such as super main, ninja, iron man and I saw him play role in our living room as those people and I though he just like monkey see monkey do as other kids until this happened. I talked to him a lot and told him that what is in those books are unreal like cartoons and do not copy from them. Now, this is similar last year but this time he saying only but still scare his friend. What should I do now Dr. This is my email address : YYYY@YYYY