Why?

I just read the news in the NYT.

This man could be my student. I am a University of California graduate. I love neuroscience.

The NRA already is fighting to control this incident, so they can keep using, and making, all types of guns in the US. Some members of the NRA are just plain greedy.

I do not have enough information to write something close to an explanation, an answer to the question of the title.

I just write some basic facts.

James Eagan Holmes is 24. He graduated with honors from UC Riverside, he lived in San Diego, before moving to Denver, to attend  the University of Colorado School of Medicine at Aurora. The NYT quotes neighbors to the effect that he was not friendly.

I remember 1999 in Columbine High School, Littleton Colorado. I saw Bowling for Columbine, by Michael Moore. Then as now, young people had rebellious behavior, then they watched "The Matrix", now the most recent Batman movie. Mr. Holmes had his hair dyed red, and was saying he was "The Joker", Batman's enemy.

Given that there are around three hundred million Americans, these incidents should be irrelevant; but:

Right now over 60,000 Mexican kids have been killed since 2006, because of the Low Intensity War going on there.

Young people are not finding jobs, Mr. Holmes has a Master's degree, and couldn't find a job. One percent of the population of the US owns close to twenty percent of the wealth. The Occupy Wall Street Movement, has tried to clarify the causes of the unease.

I have no clue, on Mr. Holmes "reasons", but I will love to offer jobs, to hundreds of thousands of young people with education, who cannot find jobs!