When I grew up in Germany, I had some pretty bad phases during my teens. I wonder if I had had easy access to guns together with lots of information and videos about shooters on the internet, maybe I would have thought about that too. I didn't have any of those so I sometimes thought about suicide but never about shooting others.

The US has a combination of SSRIs (maybe that's a factor, we don't know for sure), easy access to guns, gun culture, glorification of violence and vigilantism and over the last decades a lot of school shooters to imitate. Basically a ton of risk factors combined.