This bothers me, although we should be careful not to overstate the legal implications. As for the idea itself, I don't really get it. I mean, are we going to say that a company that makes rat poison can only go so far when it talks about how deadly it is? In the case of firearms specifically, what exactly is the line for advertising? Are we going to blame a car manufacturer that runs an ad about how its car is really fast if some teenager wrecks because they were speeding? Wouldn't that be the kind of advertising that "resonated with disturbed young men who could be induced" to drive irresponsibly? But more generally, I think it's wrong to try to blame the companies for what Adam Lanza did. To me it's scapegoating, and it trivializes his decisions and the failure of those around him to do anything. Blaming the companies lets us feel like we're doing something without looking much deeper at our society than we're apparently willing to.
While digging into this a little, I came across a pretty neat online essay about advertising bans in the United States (appropriately titled "Advertising Bans in the United States") : https://eh.net/encyclopedia/nelson-adbans/ Seems like the answer to your first question, "should we hold companies liable for irresponsible advertising" seems to be, well, we can, and we do. The four-pronged test cited in this essay, at least in theory, lays out pretty stringent rules for how the government may censor admen: is the product/activity lawful? Is the government's interest in regulation substantial? Does regulation thereof directly and materially advance that interest? Is that regulation not excessive? In practice, seems like enough wiggle room can be found to restrict commercial speech when the opportunity presents itself and where the political wherewithal can be mustered. Given the above parameters, the answer to the hypothetical "are we going to blame a car manufacturer that runs an ad about how its car is really fast if some teenager wrecks because they were speeding" may very well be: yes. Which is probably why every car commercial you see where a roadster is zooming down a country road includes the small-text disclaimer to the tune of "car being driven by professional on closed course, don't do this, dipshit." The bigger sticking point, and one you raised way back when, would be: is there any point to such restrictions? Do they work? According to this essay, at least, the answer is no. Restricting the way cigarettes are advertised, for instance, has had a dubious effect on smoking rates. Which bums me out on a lot of levels. And kind of deflates my idea. As to your final point, I respectfully submit that blame isn't a scarce resource, and doesn't have to be doled out as such for better or worse. We can shine equal light on the actions or failures of an individual's immediate community as well as his broader cultural upbringing. Both are issues that need to be addressed.