People routinely - well, at least every few months - shoot up US schools. They are radicalized online. There is a common pattern to the radicalization. However, it's ""forbidden"" to point that out or suggest restricting the supply of firearms to internal enemies of the US in any way.
Americans don't need any encouragement from foreign powers to do that. Congress has seen fit to keep letting it happen by pointing to ancient scripture about the right to develop one's own organized militia....
Difficult to be sure what would happen in a counter factual universe without foreign interference.
We do know that Russia et al sow division online as part of their anti western efforts, a strategy detailed in their "Foundations of Geopolitics" manual.
Someone commented, and I paraphrase poorly, "Imagine if Russia didn't influence the voters in 2016; all the racism and bigotry in the USA would disappear!"...