First of all, do not get PANIC. You are not doing anything wrong and for somebody else's misbehavior, you should not get affected. The whole world is a mixture of good and bad.
Now coming to the specific issue, please implement a Employee Code of Conduct. In many organizations, suddenly some people start thinking that just because they contribute towards success of the organization, they have the right to behave in any manner. However in reality, any organization is the result of its team effort, products and services, and customer service.
So my recommendation is to get buy in from management to execute a Code of Conduct policy and then take all steps to ensure that every single employee in your organization adhere to this.
Also never get in to any kind of argument with the person mentioned in your post, statements or threatening or any thing related to his behavior or your intent to introduce a new policy.
Now, to speed up the whole process, I am also sharing a free model policy template
https://qr.ae/pGNLpz