duality

• "all human beings (...) commingled out of good and evil"


  the verb "commingled" denotes a sense of closeness suggesting that there is a thin line between evil and good.
• "i have been doomed (...) that man is not truly one, but truly two."


  jekyll recognises that the duality of human nature and understands that this will ultimately destroy.
• "I learned to recognise the thorough and primitive duality of man."


  Jekyll discusses the idea of duality: he considers it a part of a man's essential nature and that duality is innate and inherent.
• "my devil has been caged, and came out roaring."

  represents Jekyll's idea that all humanity has 2 sides and highlights the theme of god vs evil. "devil" compares to satan and religion which links to the statement earlier on in the novella "distinguished for religion" which shows how both Jekyll and Hyde are somehow religious which doesn't make them seem so different after all.