Most of the people who work for you are not intentionally trying to do poor work. They aren’t trying to fail in their role, and they aren’t trying to make mistakes. Very few of them are acting on some malicious intention to do harm to the business, nor is it their intention to harm themselves.
Most of the time, and there are exceptions, people are acting out of good intentions.
That employee who called the client to explain in a somewhat impolitic manner that the problem they are having is their own fault wasn’t trying to make them angry. Their intention was to help the client see that what they are doing isn’t working, even if they lacked the diplomacy and relationship to have that conversation.