***[Set a bad example][1]*** is an  expression you may use.   

***set an example:***

>  set a good or bad example . Behave in a way that should (or will) be imitated, as in 
>
> - Dad was always telling Bill to set a good example for his younger brother, or ***They were afraid of setting a bad example for the other nations/***. [Late 1700s]

(AHD)

  [1]: https://idioms.thefreedictionary.com/bad+example