I understand why one would argue against Holmes having some form of mental illness. I get it. I really do...
That said, I don't really think that the Stanford Prison Experiment is germane to this discussion. We're talking about a guy who walked into a movie theater and opened fire (randomly as far as anyone can tell at this point) not two groups of people with specific roles to play (as was the case at Stanford). Yes, it is possible that sane people can do horrible things under certain circumstances. The unanswered question is "were they sane when they did it?" not whether or not they were sane before or after.
I firmly believe that mass shootings like Colorado are perpetrated by less than 100% stable individuals. Does this mean that all folks that suffer from mental illness are going to go out and do the same thing? Hardly. Should we be more observant about those around us (in hopes that they may get whatever assistance it is that they need)? You betcha.
Honestly I hate to see something like this happen. You're right, it does continue a trend toward stigmatizing mental illness in all of its various forms and that sucks.
One of the things that the Stanford Prison Expirement shows is that totally normal, and mentally healthy people are able to do unspeakable evil things when the right forces act on them.
We don't know if Mr. Holmes has a mental illness. What we do know is that for unknown reasons he was getting ready to quit school, and that he had access to weapons that maybe he shouldn't have. No one but him knows what the trigger was that caused him to do what he did.
What got me going on this is that post on the pavilion that talked about things like paranoid schizophrenia and what have you. Those are things that usually are bantyed about when these things happen, and who knows what's going on in dude's mind.
I just wish people would let the experts decide what's going on here.