I thought Kubo said that Humans that had committed severe crimes as a human were sent to hell, such as killing innocent people (the hollow that was a famous serial murderer as a human, he put the boy's soul into the parakeet). The Zanpakuto only cleanses sins that were committed as a hollow.
Spoiler!
still doesn't explain how the guy who killed his own wife made it, unless it's considered an accident?

