One lesson that came out of Columbine was that kids in high school need to let somebody know if a classmate is acting or talking stupidly. Annecdotally I've read of several incidents of possible violence thwarted because some kid told a teacher. Maybe that's an area where we can improve our chances. We don't want to become a nation of Margaret Hamiltons, sticking our long noses into other people's business, but perhaps we could be a trifle more observant and pro-active.
While none of the residents at Holmes' apartment complex or his colleagues at the University appear to bear any responsibility for his actions, wouldn't it have been nice if somebody had noticed his many deliveries and flaming red hair and any other goofy behavior and told somebody about it? It's true that all of his behavior (that we're aware of) can be explained away. It's not against the law to be odd. And we have a deeply ingrained cultural bias in favor of letting people "do their own thing." We're far more likely to report a barking dog than we are a guy acting strangely. Still, didn't anybody see or hear anything that raised a red flag? Evidently not.