I firmly believe in the adage, "If guns are outlawed, only outlaws will have guns." Murder is already illegal, and yet people commit it all the time. What makes anyone think a ban on owning guns or stricter gun control laws is going to stop events like the one that happened in CO? All that's going to do is make it impossible for the law abiding citizen to defend themselves, when the law abiding citizens are the only ones who will be...well, abiding by the laws.
It's against the law to walk into someone's home and steal their belongings. The law does a good job of keeping people who aren't likely to break into your home and steal your belongings from doing it.
It's the same principle, dogs, theft, murder or firearms -- or anything else "against the law."