They weren't "assault weapons".  That term is a name fabricated by the media.  Based on the information released so far, they were all semi-automatic modern sporting rifles, legally obtained, and legally modified.  Fortunately, it isn't up to you to decide who needs what type of firearms.  You are making a lot of assumptions about how many people would have died if he had bolt action rifles instead.  Personally, I have no interest or use for the slide-fire stocks that he used.  I've never seen the fascination with them, and I do agree that they probably shouldn't be legal.  But as to the rifles themselves, there was nothing exceptional or illegal about them (as far as I know from the information released so far - that may change as more information becomes available).