Dark Knight Rises killings in Aurora

[Kami]

Tighter gun laws are definitely something that needs doing in the US. There are no shops that allow you to just walk in and buy a gun here in the UK without some sort of specialist licence. There are far fewer incidents of this nature as well, though you could argue that this is because we have a population many times smaller.

Population has nothing to do with it. Japan has a much larger population than the US, yet their crime rate is much lower than the US's.
It's more of a culturaly and presonal influence than just saying poulation is the cause.
The mental state of the person is also another possible factor.