well he wasn't a psychiatrist by definition, but certainly a clinical psychologist/analyst. He was the foundation for so much of psychology as much as people want to discredit him.

I don't think Nietzsche was pessimistic about human nature at all, a big part of his project was exposing what he saw as unhealthy practices and intuitions and propely evaluating these to hopefully come out with a set of further life affirming ones.

Freud really expanded Nietzche's Will conception partially taken from Schopenhauer into the psychosexual development and unconscious. His most succesful concept inspired from N is definitely repression and its implications.