Answer
The key figures that influenced Kelsen's socio-psychological and sociological ideas during his time in Vienna were Sigmund Freud and his circle. Freud's theories on the unconscious mind and psychoanalysis, along with the intellectual atmosphere surrounding him, greatly impacted Kelsen's thinking. This exposure to psychoanalytic concepts enriched Kelsen's understanding of human behavior and societal dynamics, which he later integrated into his legal theories.