I think that the "shooter" phenonmenon has been around for awhile now, think Billy the Kid and in the early 60s, Charles Whitman:
http://en.wikipedia.org/wiki/Charles_WhitmanGranted, a lot of murderers are psychologically imbalanced and many come from abusive childhoods. I do think that the increasing levels of alienation one finds in advanced capitalist society are having their affect on social relations in general. Experiencing deep, long lasting friendships is something which is starting to pass us by as we remain busy with the bosses' work and listening to the bosses' ads and wondering whether we'll win acceptance and perhaps admiration with our displays of purchased status symbols. Cultivating friendships takes time and, if you're too busy doing boss oriented things, you'll not have time to develop them.