At first level, the ability to hit twice for half damage is much better than the ability to hit once for full. After all, at first level, the dice play a much bigger "roll" in determining whether you beat an opponent's AC. To use your example (which was incorrect), two melee fighters, one a battle monk and one a fighter, attack the same opponent. They each have +5 to hit, (18 Str and +1 BAB), and the opponent has 4 HP and 14 AC.
The fighter has a 55% chance of killing the target. The monk has a 55% chance of killing the target, two times in a row. Damage isn't nearly as important at first level as hit chance is.
Edit: This is why Two-Weapon Fighting exists (And requires a feat to even use, as opposed to sword and board or two-handed). In the first few levels, extra attacks will almost guarantee you to hit and a hit is pretty much guaranteed to kill. (With high enough Str)
The monk doesn't have an off-hand, so TWF's normal -2 penalty on both attack rolls and half Str on the off-hand attack (WotC's intended balance point) do not apply. The monk is able to be twice as likely to kill as any normal warrior because he relies less on luck.