I'm going to knee an autistic kid in school
Why are you trying to hurt this child?
Because I cannot stand his constant singing.
You shouldn't physically hit someone just because you think they are different.  In this case, it is especially heinous to hurt this child because he doesn't realize that his behavior is unusual, nor that it is annoying you.
I just want to stop him from singing. People should be able to do what they want.
You just said it yourself. People should do whatever they want. You can always ask him to stop singing for just a moment or you can leave the premises when that kid is singing.