There's not much to say beyond the obvious -- that we are not immune to crazy extremists, although sometimes we like to delude ourselves otherwise; that extremism does not grow in isolation; and most obviously, that there is no excuse for murder -- although, a la Baruch Goldstein-weapons-in-Ma'arat Hamachpelah, I'm waiting for the conspiracy theories.
I'll comment further when more details emerge.
UPDATE: AbbaGav puts it nicely:
It is too late to stop what has happened, to call back the bullets. We can only try to rise to the challenge of demonstrating how a civilized society should respond to terrorism in its midst. It is our responsibility to stop this from occuring again, especially those whose support this man may have thought he had. We must show that anyone who has even an inkling of future attacks will take action to stop them. We must show in polls and surveys that are sure to follow in coming days that we do not support the terrorist, or his actions, and we must oppose any fringe attempts to justify or minimize the act.Let me add one more thing, apropos the comments already left on this blog, and posts I've already seen elsewhere: the mob should not have killed the terrorist. But for me, that is not the story. My primary concern is what Jews do, and making sure our own house is in order. Focusing on the mob's behaviour in the same breath as condemning the killer is a form of denial; twisting this story around to make Arab mob ethics the problem here is perverse.