You don't know for sure if he was going to do it, you pretty much defended yourself from him but he never tried anything yet. I would of probably done the same thing, but still, you shouldn't be fearing for your life over a little brother who sounds like he is 11, 12, 13?
Me and my older brother have done so much shit to each other no one would even believe me here if I told them. I know what it feels like to be your little brother because I used to be like that when I was younger, he said that he wouldn't stab you and that's pretty much like daring him too, because he knows that if he doesn't do anything now then you'll make fun of him for it.
Just tell your parents about it, let him get in trouble then he might think twice before he goes crazy. He'll grow up eventually.