Motive, deranged, insane, bath salts, comic books and video games; people where mean to him, perhaps,.....whatever. You can search the best minds in the world and they can not tell you what caused this. Of all the knowledge of man, no one has a better answer then the bible, and the biblical answer is the best answer for anyone looking for answers in this tragic case.
We live in a fallen world, where sin and evil inhabits our existence and permeates the hearts of mankind. "12 Therefore, just as sin entered the world through one man, and death through sin, and in this way death came to all people, because all sinned." Romans 5:12 (NIV) ..."For all have sinned and come short of the glory of God" Romans 3:23 (NIV).
Sure this James Holmes might have, what we call, "issues", but his issues are firmly entrenched in his own sin, which he has embraced for whatever reason. We should not be so bold to single him out as any less then us, because "If we say that we have no sin, we deceive ourselves, and the truth is not in us." 1 John 1:8
I have heard more of the victims being interviewed in the hospitals as they recover, saying they feel sorry for this man. I find that encouraging. We should all Love this man, not for what he did, but for for what he does not have, the love of Christ in him. Who will show him that? Who can? If Christ works through his people then it should be his people.
"Nothing in all creation is hidden from God's sight. Everything is uncovered and laid bare before the eyes of him to whom we must give account." Hebrews 4:13 (NIV)