Assault weapons are to blame for many of the gun massacres that occur in the United States. Specifically, they are to blame because they are way to easy to get a hold of and they can cause so much death and destruction with very little effort. Getting rid of them won't solve everything, but it would be a great start.
Assault weapons are not the only thing used in mass murder. A dedicated individual could kill large numbers of people with any type of weapon. If the criminal is going up against unarmed citizens he can use any weapon no matter how simple or weak. Hunting rifles have the potential to do more damage because they are a larger caliber.
Blaming assault weapons is blaming a symptom, not a cause. Those mentally disturbed enough to cause a massacre are going to do so whether they can easily get assault weapons or not. A better, more long term solution is to treat and educate individuals on signs of mental instability. More restrictions on assault weapons is a start, but a greater problem needs to be addressed.
I do not think that you can blame assault weapons for massacres. Guns do not kill people, people do. A gun is only as strong as the person using it. If the people who did the massacre did not use a gun, they probably would have used some other type of weapon.