There appear to be three classes of mass shooters:
1) Total lunatics. The recent Aurora shooter and the guy that shot Gabby Giffords in Tucson both fall squarely into this category. Tactical knowledge tends to be the lowest of all categories. I suspect this class is also among the more common types of mass killers.
2) Racists or other bigots. The recent Sikh Temple shooter is of this category, as is that jackass in Norway and various "Sudden Jihadi Syndrome" types like the military shrink who shot up a base with a handgun not too long ago
. These types are "sane" by the standards of most courts and are capable of very methodical planning - the Norway case is a horrific example. I consider them extremely dangerous, probably the most dangerous category.
3) "Specific grudge cases": the shooter has a personal grudge against at least some of the victims...see also the Columbine shooters, and a LOT of cases where a guy goes after his "ex" and takes out others as well
- or the sort who goes back to a former workplace to "get even". The level of planning seems to be (on average) somewhere between the above two types. (Some of these sorts are actually crazy, others...not so much.)
Mental deficiencies among the "not totally crazy" sort are common - usually either anger management issues from hell, or something in the sociopath/psychopath spectrum. Or...well, they're just plain a-holes - meaning, usually, "not raised right", "spoiled brats", etc. There really are people in the world who expect to "get their way" at every stage of the game and get furious when it doesn't happen.
What else..."anger management"...fancy way of saying "somebody who enjoys the feeling of both power and loss of control that unleashing total rage entails". These people enjoy the "incredible Hulk" feeling of going totally bellowing out-of-control nuts in what they see as "justified rage". I detest this sort of so-called "human" more than just about any other. It is NOT "insanity" in any sort of fashion that might excuse horrible (or especially murderous) behavior.
The true mentally ill sorts (such as the recent Tucson shooter) have my pity. In that case it was genuine schizophrenia...the videos he made before the killing are absolutely tell-tale. I am not in favor of killing that nut - it wouldn't be right because we might one day be able to really cure that and even that aside, we need to study critters like this (and I suspect the Aurora shooter is similar) and you can't study the dead near as well...
For the record: I'm not completely mentally normal myself - I'm a mild-ish Asperger's case. But that isn't linked to either violence or loss of judgement and anybody who knows me will tell you that if anything I don't get angry enough