Whilst never having seen a spirit being I have certainly encountered their work, presence and persona in the Pharisees (religious leaders).
I find the "militant ignorance" and "malignant self-righteousness" of the Pharisees to be powerful evidence of the work and thinking of evil spirit beings.
I do not believe we humans are able to achieve the extreme levels of fanaticism, fundamentalism, sectarianism and irrationality which we have, completely unaided.