Until yesterday afternoon, many of us probably thought of security guards as annoyances. That's if we thought of them at all. They're ubiquitous in Washington -- checking our IDs, telling us to empty our pockets, directing us through the magnetometer -- such a constant presence that they can become invisible, just another extension of the security apparatus that's attached to nearly every aspect of public life these days. And then suddenly something happens. A man tries to get into a museum. He has a weapon. There's shooting. Yesterday, Stephen Tyrone Johns of Temple Hills died. He was shot at the...