Much of the public rhetoric surrounding the Orlando massacre is muddying the moral waters, making it harder for us to address the threats we're facing, never mind establish justice and find healing as a society.
Evil consists in regarding or treating a human being as anything less than a person—an absolutely unique and unrepeatable, free and responsible, self-determining subject, an image of the Most High, made from love and for love. When we reduce ourselves or another to an object of contempt or manipulation—a means for our pleasure or profit or prestige or relief, in big matters or small, we commit evil.
Murder and rape, torture and terror are some of its most extreme manifestations, but its seed is a disposition of the heart—the same disposition ("you are nothing; you don't matter") that induces malicious gossip or bullying or cheating or stealing or self-abuse. That's why Jesus could say that anyone who hates is brother is a murderer, and a man who looks at a woman lustfully has already committed adultery in his heart. It's why Satan can be variously referred to in Scripture as "the father of lies", "the prince of darkness" and "a murderer from the beginning."
It doesn't mean that all forms of evil are equally grave; it does mean they're of a piece, spiritually speaking. They spring from the same root. Good acts and gestures too, small and large, are of a piece—they are born of concern for the rights and dignity of concrete persons, self and others.
It's not evil to hold that certain acts (like lying or adultery or forgery or theft) are objectively immoral. It's not evil to defend myself or others from assault. Guns are not evil. Believing that my religion is true and others false is not evil. Holding that marriage can only be formed by a man and a woman is not evil.
Dehumanizing or demeaning others, though, never mind killing them because they don't conform to my will or my belief system, is evil. And any ideology that calls for or permits or praises the destruction or abuse of human beings is an evil ideology.