Let's say that guns are "just" a tool. Ok, but that does not mean we should not ban assault rifles. If a tool is primarily used for evil then of course we should ban it. Zyklon B was used to kill millions of Jews during WWII. Sure, you could argue that it was just a "tool" too but we still banned it because it was a "tool" used for evil.
Assault rifles are weapons of war and in civilian hands they do nothing but cause death and destruction. They are a "tool", but they are overwhelmingly used in mass shootings. Thousands of deaths every year. It's time we do something about the assault rifle problem in this country.
Just because we have the Second Amendment doesn't mean we have to sit by and do nothing. Guns can be regulated, and banning assault rifles doesn't violate the Constitution or anything like that because you would still be allowed to own other guns. Banning gun owners won't be easy but it must be done.