That fucker in CO would have obtained those guns regardless of their legality.
The second amendment isn't just in place so that we can hunt or carry concealed etc, so the question 'why do we need assault weapons' isn't even relevant to those situations. I've never needed an AR15 to take out a whitetail deer, and I did it with a single shot bolt action 270. The second amendment gives us the right to keep and bear arms, because there is a chance we'll have to use them to overthrow a tyrant government (again) and I'm definitely not going to be shooting back at fully armed oppenents with a fucking deer rifle.