I include the metta practice every day in my half-hour meditation sit. The idea is to send out goodwill and wishes for true happiness to one's fellow living beings everywhere. Goodwill is hard to send to those who have done harm, but to harbor ill feelings and animosity is only to retain the toxins in my own heart. The wish for true happiness is not a glib one: "true" happiness, as I understand it, is not about wealth, or material success, or "getting one's way," but rather about being at peace with oneself and others and in harmony with the universe. Those who experience it, then, will not intentionally bring harm on those with whom they share the planet, but rather share the real wealth of well-being.
So it makes sense to send metta to those who lost loved ones in the Aurora massacre, and to the survivors who will carry the physical and emotional scars in the months and years to come. But it also makes sense to send metta to the perpetrator; and to all those who feel the need to arm themselves with deadly weapons; and to all those who clamor for their second amendment rights, and the politicians who support them. May they be guided in their actions and intentions by wisdom and compassion. May they find peace in their lives.
And if this sounds a little pollyanna-ish, so be it. I believe we human beings would all be better off if we were able to find true happiness and peace in our hearts. If we could all live without causing harm.