"you knew I was a snake when you took me in".

I've read traditional African folktales with this theme [snake asks farmer or someone else to hide him from some other animal; farmer does so; snake bites him; when farmer asks the snake how could you do this when I helped you, the snake responds with those words...moral: be careful who you befriend].

I wouldn't be surprised if there are folktales from African American culture * that Oscar Brown Jr. may have heard-with that same theme.

* and/or other cultures for that matter...