As has been said the difference is a bomb (for example) requires time, a knowledge of explosives and there's a decent risk of it failing. That means it's impossible for someone to snap and just make a bomb, you can however just snap and grab your firearm.
The problem with the American mentality is they often talk about criminals as if they are a different species than the 'good guys', they don't think that anyone, no matter how good they are, can just snap and go postal at any time without warning.
Yup, its like crazy difficult to grab alcohol, cloth/rags and a lighter.... oh wait, its not.
Also your assuming that the majority of these incidents aren't premeditated and planned.
"Impossible" to snap and kill people en masse easily without a gun is a massive over reaching statement that is patently false.
They use guns in the Us because they have access to them, if they didn't, the people who go crazy and decide to kill people WOULD use other weapons and be JUST as successful, maybe more so.
Imagine if this guy didn't have a gun, and decided to go to his local gas station, fill up a bunch of gas cans, go over to the school, set alight to a gas can at every exit and watch the entire building go up with EVERYONE inside being killed.
People are focusing on the guns because they are there and thats whats used, the guns don't cause the crime and there are a ridiculous number of other ways to kill people without much planning. Someone could simply drive through a fence and over dozens of kids on the playground, petrol in a smallish bottle taken into an office building, someone sprays it around a bit and in stairwells and sets a fire... is unfortunately ridiculously easy and could be incredibly deadly. With an office block a single man with a gun would be hard pressed to go floor to floor and murder everyone in a 20 storey building, with 30 seconds he may be able to start a fire that kills everyone though.
We like to PRETEND that without guns, these tragedies wouldn't happen, because we like to PRETEND that it isn't true that anyone we ever bump into could be a crazy guy on a bad day who could kill us and/or everyone we care about.
If we demonize guns, because we don't have guns in the UK, WE feel safe because we PRETEND the chances of mass killings is stupidly low as it would be really difficult without guns. The quite un-nerving, scary reality is, anyone can kill a bunch of people anywhere with very little forethought and with relative ease.