There is almost nothing one can say regarding the tragic murder of 20 children and at six adults in a school in Connecticut. I have no doubt we will eventually learn what motivated this young man, and I have no doubt that nothing can ever replace the children he has killed, or assuage the grief of the parents and others affected by it.
How does one prevent these events? There is no easy answer. We cannot "un-invent" guns and even where there is tight control there is still abuse. That leaves only the question of what kind of person can commit such an evil deed? The weapons this young man used did not select the targets or pull the triggers or load and reload - he did. Had he not had access to his mother's weapons, he would probably have found some other weapon to use.
All any of us can do at present is pray for the departed and those left to mourn.