What legislation could have possibly prevented this?

Yep, not disagreeing, I just thought a qualifier wasn't a bad idea so people don't equate "mass shooting" or even injury to the quoted sites "school shooting" number of 18.

Edit: It is still fucked up though, guns have been a part of this country since the beginning but our society has so many sick people in it or itself has become so sick that this type of thing is happening on the regular ... 

Well, we don't actually know much of anything about what happened so at this point, it is hard to say.

That said, gun control proposals generally focus on broad efficacy, not one singular event. It will never be possible to prevent every instance of violence, but it may be possible to reduce gun violence broadly by implementing stronger restrictions to who can own firearms and ammunition, how individuals purchase firearms and ammunition, and how law enforcement approaches issues regarding firearm ownership.

As a gun owner, I am ok with stricter background checks, even for people at gun shows.  I'm also ok with limiting a magazine to 10 rounds.

Beyond that, I can't say I support more restrictions.

Well, universal background checks are one of the most important proposals for reducing gun violence. Beyond that, I'd say the main things are preventing people from purchasing guns who are in populations with a high risk of committing violent crimes, such as individuals with violent misdemeanors and substance abuse charges. Looming over all that is improvements to the infrastructure that these systems are all based on, including improving enforcement of the laws on the books and making sure all the appropriate information is available to the background check system (earlier this week, there has been news that 4,000 individuals were added to the system from military dishonorable discharge ranks who should've been on the books a while ago after a shooting last year brought attention to it. There are a lot of names missing from the system from many sources, but things are slowly improving).

Why haven't the states banned guns? Guns were banned in Australia in 1996 and they haven't had a mass shooting since. If I ever have kids I would never let them attend America schools. No wonder they call Australia the lucky country

Sure, I agree, we need a better computer system to screen people before they get approved to buy a gun.

It's part of the bill of rights. 

"Why haven't the states banned guns"

The reason is the second amendment of the US constitution.  The US is not Australia and never will be.

Could you educate the poster on what the bill of rights states?