Let's not go too far here. The original post's logic might fail because it's a broad-brush argument, is simplistic, & uses armchair psychology to boot...but there is a grain of truth in it. Many of those school-shooters were indeed socially awkward; it's not far-fetched to say that the girls in their community considered them undesirable & that such rejection can be a factor in causing a killing spree. Sure, most unattractive nerds don't go on killing sprees, but not everyone has the same tolerance level for social ostracism.
One thing needs to dealt with but is rarely ever discussed: genetic factors. Ever since the Human Genome Project, hereditary predispositions for various illnesses have been discovered & more discoveries are made every year, if not every week. Even before that it has long been known that depression, manic depression, & certain psychoses tend to run in families & skip generations in regular patterns. If society is to eventually rid itself of crime, it's not only poverty, lack of intact families, weapons access, etc. that must be dealt with. Bad chromosomes must also be weeded out of the gene pool (through civilized means!). Today's technology allows a single lunatic to become dangerous to many people or even an entire city...we can't wait for natural evolution to take care of the human condition.