set a bad example

To do something or act in a way others should not emulate; to act as a poor model for others. John, please don't curse like that when you're angry—it sets a bad example for the kids. Don't set a bad example for my employees by arriving late to work or avoiding problems when they arise.

set an example

To do something or act in a way others will or should emulate; to act as a model for others, good or ill. John, please don't curse like that when you're angry—it sets a bad example for the kids. I try to set an example for my employees by always arriving to work on time, replying to emails and phone calls promptly, and taking care of problems as they arise.