I've always been sceptical of the role played by the legality and availability of guns in the overall murder rates. People often compare the British murder rate with the American rate, but the thing to note is that the difference between the two countries was apparent long before the UK had any gun control laws. In fact the high levels of violence in the USA's southern states goes back to before the States was a country.
It also ignores the fact that plenty of countries with restrictive gun control laws have even higher murder rates than the USA, such as Brazil or Russia.
However it seems pretty clear that the one exception to this is with killing sprees by previously law abiding individuals who end up killing themselves at the end of the massacre.
A killing spree differs from regular crime in two key points- the shooter usually commits suicide so isn't deterred by any repercussions of their act and also it is usually done by otherwise law abiding people. The fact that they don't have to worry about consequences means that the usual disincentives for crime don't apply. Because they are otherwise law abiding they would not have access to firearms if they were illegal. This makes killing sprees much more likely if guns are readily available.
This isn't to say that gun laws are the most important factor in massacres or even that we should definitely have tough gun laws (there are some advantages to gun ownership too). However insisting that the availability of guns is somehow irrelevant to gun massacres seems to put ideology over evidence.