[Hey, at least he knows about both those things too...]
Uh, no? Because an assassin is by definition someone who kills in secret, and usually that's because a) they're doing something illegal or b) they're killing someone they shouldn't be killing. Knights are basically the same as soldiers, so they're acting on the orders of whoever's ruling the country and that basically gives them the right to kill whoever they're ordered to kill and it's not secret or illegal.