This is a great rule of thumb for employees and managers both: neither side should ever be too surprised. Whether it's performance evaluations or firing/quitting decisions, let the other side know if you're unhappy, and talk about it regularly (at least, one-on-one). Every time I haven't talked to managers about this sort of thing, I've regretted it.

If you're leaving a company, you might not have told the manager that you were interviewing around, but you certainly should have talked to them about being unhappy.

If you're firing an employee, they should have known (almost as long as you've known) that there was some culture mismatch, or that their performance wasn't meeting expectations, or whatever.

There are of course always exceptions, where you think someone is crazy or an asshole, but most people are reasonable and can handle hearing that things aren't great.