I think it would be cool to see a sect of religious fanatics who let themselves succumb to radiation and mutation. They would be always aggressive and would carry melee weapons and on rare occasion a firearm. They would be attacked by zombies, too, since the zombies view them as a human just like our characters. They would have random mutations, bad or good. I just think it would be cool seeing a hostile human. They could spawn as random events, much like supply crates or dead soldiers.
For flavor, they could also spew passages from their chosen religious dogma.