Just because the Aurora shooting and other recent shooters didn't use automatic weapons, doesn't mean they couldn't acquire them. I would speculate that the choice of semi-automatic was a specific choice. He wanted to pull that trigger every time, it was very personal for him. If his objective was to kill as many people as he could, he would have been much better off using a bomb. So it's not that the shooters COULDN'T get an automatic weapon, it's that they didn't want one. My point it, criminals, psychopaths, will get what they want, illegal or not.
Also, your point that if there were smaller magazines, there would be less dead. That is total speculation, it sounds nice, but there is no proof that would actually work. The fact is, as terrible as these shootings are, they are very rare. We can't give up personal freedoms because a ban on high cap magazines could possibly prevent a shooter from getting one which could possibly add a second or two to a mass shooting which could possibly save a person.
Finally, as a point not specifically to the magazine issue, but rather to the gun control, here's one of my favorite quotes:
They who can give up essential liberty to obtain a little temporary safety, deserve neither liberty nor safety." Ben Franklin