Technically you can see yourself at a distance of 0, so is it legal to shoot yourself? I can think of a few situations where this might be useful:
- You're an outlaw with 1 life left, and the Sheriff is on your left and will surely kill you. By killing yourself you prevent the Sheriff from getting 3 extra cards
- You're taking a Ghost Town turn, and you have a Bang card and a Dodge. If everyone you want to shoot is out of range, then shoot yourself, use the Dodge, and draw another card in the hopes you get Indians or Gatling or something like that
- You're Bill Noface and you want to draw more cards on your next turn
And I'm sure there's more...