Thanks for the advice, everybody. I had been documenting warnings and misbehavior by the employee. The firing has occurred. Held on to final paycheck until all company equipment was returned in working condition and I could terminate all access to customers and data. I was more so concerned about retaliation after the fact.

Careful calling anyone mentally unstable or anything similar. Often these are protected classes and need to be cared for rather than terminated. I presume you are not an expert on mental stability, but you have some experience seeing that they are not achieving the objectives you’re setting for them. I’d focus on the latter as much as possible. Of course, employment attorneys are very important to keep close in situations involving terminations, and if you can sleep well terminating this employee and feel they are better served on unemployment or at another firm, then follow the rules, the law, pay them, see about getting them to sign a release, and then move on with your business and try not to hire someone similar in the future. Good luck!