Yeah, I think the isolation + fringe opinions part is key. Lord knows there is a niche rabbit-hole for every bizarre desire and destructive emotion on the internet. Micro-communities can now easily form around fleeting ideas and phases of despair, giving them a place to take root and grow. Then add the intersection with gun culture and you're baking a cake.
I've had similar thoughts to mk in that what used to be suicides are now turned outward. I'm not sure that the hopelessness or despair have gone up though. I think it's more that those with that despair have access to networked micro-communities that celebrate taking others out with you, they see the media reward others who have, the constantly growing list of others who have done the same, all against a backdrop of a culture where the NRA add below is mainstream for large communities in our country: