I'm going to badmouth a co-worker
Why would you want to badmouth a co-worker?
I don't like him and I want to ruin his reputation.
You shouldn't say rude things about a coworker. If others find out, you may be fired from your job.
He can't fire me, I'm the best employee there. I would get hired anywhere else. I'm the one that's going to get a severance package.
Unless he's done something illegal, I can't see where it makes sense to badmouth him. Plus it's mean. If you do something like that too often, you won't be considered the best employee for long.