A manager's job is to coach and give feedback so clearly that firing someone is never a shock. Vague "fit" excuses invite lawsuits. Document everything, have multiple conversations about what needs to change, and termination becomes the logical next step, not a surprise.
Nobody should ever be surprised when they get fired. It's that simple. They should either have done something so egregious that is an obvious reason for termination, or you should have had multiple conversations leading up with them to the termination that let them know where their opportunities for improvement were, where they were missing the mark. You should have been giving them training, coaching, feedback along the way to to help make sure they had every opportunity to perform the role that you hired them to do, or to help coach them up and find the right seat on the bus if necessary. Too often we see people say, "Oh, they're just not a good fit," way too early in the process. They end up terminating someone for a wrong reason and ultimately end up with compliance concerns and wrongful termination suits on their hands. So, make sure nobody is ever surprised when they get terminated from your organization. It's the right thing to do.
