'Zombie' is a nickname I got while I was in the service. If you'd ever seen me after a night of partying, but before coffee and shower, you'd understand.
When I started playing online games, I tried using 'Zombie' as my screen name, but it was always taken. It always suggested adding a number, but I didn't want to be 'Zombie68375' so I figured, if it's going to be a number, then why not Zero?
Hence, Zombie Zero.
or Zom, Double-Z, Zed...