Greentexts, memes, everything 4chan.
There people who are literally bordering death by hunger and drinking dirty water probably from the moment their embryo began to form. Malnutrition is proven to led to low IQ. But they all don't go on killings spree, otherwise there would be no-one suffering from hunger anymore.
Malnutrition is proven to led to low IQ.
Malnutrition, pollution (particularly lead), chronic anxiety which often manifests as a consequence of abuse, elemental exposure which can happen during heat waves / cold snaps in homes without good insulation or AC/Heating...
It all contributes to degraded cognitive functions, particularly early in life. They also contribute to chronic pain, which can lead to substance abuse. The oxycotin-to-heroin-to-fentanyl pipeline is filled with people who lose access (or never enjoyed access) to traditional medical care, didn't have access to preventative braces and surgeries (dental being a big one), and who suffered workplace injuries they couldn't treat.
All this shit is fixable and preventable. But it requires materials, labor, and some degree of competent administration to achieve. Westerners have been indoctrinated into believing none of this is possible. Its all too expensive. It's all too difficult. There's no one available who you can trust to oversee these projects.
So the problems persist in some of the wealthiest corners of the world that dirt-poor countries like Cuba and Vietnam resolved decades ago.
Determinism is a helluva drug...
But seriously, we gotta look at the structural issues at play in things like this. A lot of gun control advocates want to point at things like this, and say, see, this shit doesn't happen in any other country in the world, obviously the guns are the problem, we need to take the guns! But when you look at other countries, you see that the US is one of the few that has both guns and almost no social safety nets. The proposed solution on the left is never "fix the problems that lead to crime and violence"; rather, it's "take the tools used to commit crime and violence"
You can use the facile response of saying, no, this is all on Cruz, he made a choice. And it's true, kind of, except that his choices were significantly constrained by the social conditions that were imposed on him. That's not saying that what he did was right, nor does it make it the 'fault' of the people he victimized. But things like this simply do not happen in a vacuum.
Here's another way of looking at it.
If you take a dog as a puppy, starve it, leave it outside all the time, ignore it, kick it, never give it any positive interaction, and always punish it for any 'infraction' when you've never put in any time to teach it better, are you going to be surprised when it mauls someone? Why do you expect any other animal to behave better than a dog just because it walks upright and has no fur?