When these tragedies strike the question, we ask always is why. We delve into the life of the murderer and try to figure out for our own sake why this happened. Why do people ever commit such acts of darkness? How can we avoid such acts that miss the mark and intent of God's design? At that point of God’s intervention, we lose our ability to choose and follow God as a human and become puppets. The better question to ask is what? What is God doing in the responses to this? Mercy, grace, and compassion, and so we respond, in kind.
Further the next question is how to prevent it... shootings require a shooter, and the origins start in there. That person's soul, it's soul wounds, and it's delusions that are a product of the lack of God's presence, love, mercy, and grace. People don't get to those places of evil in a void, they are deeply broken. Never will this be an excuse or justification for this horrible act; but perhaps it may serve as an explanation to the depth of need people have for a savior.
The best we can do now is stand against divisiveness and bring unity, to our immediate circle of friends and pray for all involved that night. We cry Mercy for all.