If a person is the type predisposed to violent tendencies, they will find their motivation in their mind. The least little thing can set them plotting. Usually, not anything that would stand out to a normal person. Actually, more often than not, if you've ever been around a person like that, you're in the dark as to what brought on their current bout of anger. The reason for this is it's something they created from their own warped sense of the world. They're not healthy, they don't react to stimuli like a normal person would. They don't think rationally, and they don't even usually think similarly to another person predisposed to violence. One person can think the government is out to get them and decide to blow up a building with a truck full of explosives to retaliate. Another can think the world is too evil for their family to continue living in and murder their wife and children. Another may see demons in their child try to carve them out. Yet another may think their parents are plotting against them, destroying their hopes for a future, and murder them and everyone they think was involved in the imaginary plot.
It's not rational. They don't think rationally. We're not going to find a rational reason for those types of situations. I'm sorry, but we're just not. Even if I do understand the motivation to try and find one.
Yes, if they have access to weapons there's a chance they're going to use them in an act of violence. If they've been trained to use them, there's an even better chance that they may use them against innocent people.
Where does this leave us? I don't know.