To find Brad as a zombie in Resident Evil 2, the player must select the Normal difficulty and reach the R.P.D. building without picking up any items or weapons on Scenario A (Leon or Claire). The downside to this is that you will miss out on the Shotgun for Leon S. Kennedy, or the Bowgun for Claire Redfield early on; this isn't as much of a loss for Claire, as her special weapon lacks power, and there are two of each in game. You can also fight Brad in Scenario B, under the prerequisite that you encountered him in Scenario A.
You receive a key for killing him that will allow you to change the costume of the player character. Claire will also be able to find a second handgun if you equip the cowgirl costume.