From shy teen to neuroscientist to sadistic shooter: "Batman" killer had epic decline

James Holmes was becoming unhinged in the months before he dressed in a gas mask and body armor and began shooting at a packed midnight "Batman" premiere.

Prosecutors say the 24-year-old neuroscience graduate student told a classmate he wanted to kill people. He also threatened his psychiatrist, and began massing an arsenal of weapons, including thousands of bullets and enough chemicals to rig his apartment into a potentially deadly booby trap.

His trial starting Monday could finally help explain why a promising scientist-in-training would kill 12 people and wound 70 more during the July 20, 2012 attack in suburban Denver.

But experts say anyone looking for a single trigger or tipping point may be disappointed, since mass shootings often mark the end of a killer's long decline.