I think one overlooked aspect here in the "gun/mass killing culture" discussion is the ubiquitous presence of media. Were there mass shootings 50 years ago? Yes, there were, but there were far, far fewer methods by which you could learn about it. You had a handful of networks on TV to tell you about them and then the newspapers and magazines. And that's only if they wanted to tell you about it: if they thought it was important for you to know about it.
Compare that to now. 24 hour "news" networks, a computer in your pocket/purse at all times, minuscule moments of downtime filled with someone checking out their facebook feed or whatever on their phone. You have 24/7 access to news no matter where you are, from a hell of a lot more sources of it who all have both their opinions on what you should know and also their own profit motives. Sex sells. Violence sells. Controversy sells. It all gets more eyeballs and ears which fuel that profit motive. So when a school shooting happens, what follows? Ubiquitous coverage, lots of outlets digging for every last detail to feed their rabid audience that wants to know about the shooter and the people who died. They keep it alive by using it to talk about gun control, and what could be done to stop this horrible tragedy. It makes one really question whether or not they want to stop it from happening since it's covered so breathlessly and so completely because there's an audience that craves it. But to the original point: the fact that it happened is shoved in your face and smeared around for days on end: it makes it feel that much more prevalent and that much more of an issue because, as stated, overall homicide is down.
Furthermore, if mass shootings were really our culture, if it was "normal", then no one would pay attention to it cause people don't really care about shit that's normal: it's the abnormal that gets people's attention. People see that and go "wow, that's crazy, i need to know more." If you want to talk about culture, if anything, it's that our culture is obsessed with watching/consuming violence but not committing it. These school shootings are universally condemned as horrible and terrible and tragic but that doesn't stop us from consuming the aftermath of it. We play violent videogames, see violent movies and tv shows and see the aftermath of actual violence on our TVs/devices every single day. Those who aren't directly affected by a mass shooting can't appreciate the impact of it because it blends into the tapestry of violence we consume daily. So if anything, our culture is one that doesn't condone or encourage violence against others but is quite eager to consume the fictitious depiction of it and the quite real aftermath of it.