I really commend you for asking for advice on this and keeping an open mind to your special needs children’s differences.
I get the sense, from the way you talk about this incident, that you are reading way too much into the words that were spoken. When you hear the words, “I hate you”, it means something to you that it does not mean to this boy. Rather than say, “I am really shocked at what comes out of these kids”, which implies a moral judgement about what’s happening, I would encourage you to say, “How do I help this child manage his strong reaction to this boy in a socially acceptable manner?”.
First, it’s difficult for kids with Asperger’s to express themselves, and difficult to properly explain how they are reacting to people. They often have strong visceral reactions to people that are based on one of two things. A) the boy may have excluded him at recess or done something else that he didn’t like, and he doesn’t know how to react to it, or B) the boy may have something about him that is setting off a sensory sensitivity. For example, the tone of his voice might be unbearable to him, or the color of shirt might cause a headache. It might be hard for you to imagine how another person could possibly not be able to bear something as innocuous as another person’s voice, but that is what the world is like for *some people *with Asperger’s.
A good way for you to handle something like this in the future is to calmly and clearly enforce the classroom rules for respecting others. You might say, kindly, softly, and calmly, “John, what you said to Billy was disrespectful and hurtful. Do you understand that?” Let John answer. You want him to practice the ability to reflect upon and gain insight into how he is perceived by other kids when he says certain things.
However he responds to your request for him to reflect on how his words made Billy feel, whether positively or negatively, say, “I need you to apologize or there will be a consequence.”
Later, sit down with him (with a parent, maybe, or a classroom aide, or whoever is assigned to assist) and talk to him about what is causing his reaction and how to manage it socially. Problem=solution. That’s the format you want to use, and you want to do it an entirely genuine and non-judgemental way, without sitting back and imagining what a terrible child this is. It truly is a difference in the way he’s perceiving his environment, and he has to be taught, directly, how to interact with people because of his condition. It has nothing to do with him being “bad”.