The bleach didn't kill them it just stung them in the same way holy water strings demons. Nothing they used actually killed them. Decapitatig them left them vunurable for hours but left alone the head would reattach itself. They started locking their heads up away from the bodies so they wouldn't heal but they were still technically alive.
It eroded them. They would have most likely recovered, but it was more effective than holy water was to demons.