If I ask my employer if I can start a YouTube channel and then after getting permission I start posting videos that make the company look bad, and showing how awesome competitor products were I wouldn't be surprised if I got fired. I wouldn't be fired for the account but what I did with it.

We don’t know if the person did any such thing (bad mouth the company or its products).

However, in the scenario they did, then the company would have grounds to fire them.