You don't have to be a sociopath or a criminal to be able to do harm to a company. There are some positions where clients loyal to the person rather than the company like sales or some form of producer/manager depending on the industry.

There are stories of sales teams going in and grabbing their rolodex (instantly dating the stories) so they could use the contacts where ever they land next. They could also reach out to the contacts to attempt to poison them towards the company, libel be damned.