How to deal with the office nut-job

Whenever possible in casual conversation, say something like "remember, I was always nice to you."
Target someone you both dislike and try to fan the flames of that hatred to defer anger from you. "That Peterson is such an ass. If he did to me what he did to you on Tuesday- well, I don't know what I'd do."
Use the aftermath of the tragedy to your own ends. But remember to make your if-that-crazy-dude-shoots-up-the-office wish list well in advance. "Sir, I don't know what this means, but I think Rick was really upset yesterday about the Internet connection. Perhaps we should upgrade to a T1."
Sprinkle water cooler chatter with solid psychological counselling. "These voices you hear, do they tell you to do things?"
Search through his browser cache when he's at lunch and see if you can find any celebrity photos. If he has an obsession with a specific actress, mention to him right away how much you love her and how you would love to impress her some way if you only knew how.
If you feel you can broach the subject lightly, attempt to enter into a "if anything happens to either of us, the other one erases all the porn from their computer" pact. That will surely spare your life.

Don't:

Refrain from insulting his taste in software or operating systems. "PeachTree Accounting? Good God, man. Only pussies use PeachTree."
When he shows up with fifty clips of ammo to kill an office of fifteen people, do not criticize his lack of efficiency.
When arguing with this person (and you will find yourself in this position no matter how hard you try to avoid it), do not say things in anger like "what are you going to do? Mow me down in a hail of bullets?"
If your office does have a "when will the guy freak out" pool, do not offer to administer it. If you must participate, make your guess years in the future. It will be five bucks well spent.
If you own a t-shirt with a targeting reticle on it, do not wear it to work, even under other clothing.
Do not (no matter how tempting) play any kind of practical joke on this person. That includes leaving messages from "Satan" in his desk drawer. "Bob, have you killed everyone yet? I'm waiting! - xoxo, Satan."

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