Making a "true" bot for Halo (Xbox) or Halo PC is not possible as the engine doesn't support fake players.
The engine DOES support spawning AI monsters (from the single player game) in multiplayer, but there are some limitations (like they won't respawn when killed, etc).
Search on the
www.halomods.com forum for "bots" and you'll find plenty of information (although a LOT of it is incorrect information because 99% of the people on that forum have no idea what they are talking about).
botman