FWIW, it is incredibly difficult to get children permently excluded from schools. It's not taken lightly either and I don't believe the school have taken it lightly. t takes weeks and weeks of paper gathering, meetings, Govenor meetings etc etc, before a final exclusion meeting makes a decision.
They are on the inside, not you and whilst you might feel you have the right to know the facts, you don't. It'll just end up being a witch hunt. You have no idea about this students background, his reasons for 'stabbing' the other student or indeed his educational needs.
No, it isn't particularly fair on the other students and you're right, adults would not put up with this behaviour in the work place, but every child has a right to an education and that, I'm afraid, is that.
You also might also be interested to know, that most schools will have to have another student join for a 'fresh start' in the place of the departing child. Lord only knows what that one needs a 'fresh start' for. He/she will have been turfed out of their school for something...