Here are some possible antonyms depending on the context:
If you're referring to actions:
* Inaction
* Passivity
* Stillness
* Quietude
If you're referring to conduct:
* Misconduct (but this is more of an opposite in the sense of being bad conduct)
* Decorum (this is a formal opposite, meaning proper behavior)
* Propriety (similar to decorum)
If you're referring to personality or character:
* Nature (this is a more abstract opposite, referring to inherent qualities)
* Disposition (similar to nature)
* Temperament (similar to disposition)
It's important to consider the specific context of the sentence to determine the most appropriate antonym for "behavior."