Also, forgot a few points:
1) You can build guns in your own home with not a ton of difficulty.
2) Most serial killers and mass murderers do not use guns- serial killers tend to favor strangulation, stabbing, and poisoning as their modus operandi, and most mass murderers favor explosives.
3) One of the most effective bombs is a standard 25 gallon propane tank- available at your local gas station for under $50.
4) Building bombs, making molotov cocktails, and mixing napalm are not difficult processes, and walkthrus are available online.
5) Death by strangulation or by beating may expend more effort, but are not terribly difficult all things considered. If you are angry enough to shoot someone and have that little control over yourself, you are angry enough to kill them in any way possible.
6) A good kitchen knife will slice through bone like hot butter.
7) Anything can be made into a weapon by someone with some ingenuity. ANYTHING.