Stalin was ruthless, temperamentally cruel, and had a propensity for violence high even among the Bolsheviks. He lacked compassion, something Volkogonov suggested might have been accentuated by his many years in prison and exile, although he was capable of acts of kindness to strangers, even amid the Great Terror. He was capable of self-righteous indignation, and was resentful, vindictive, and vengeful, holding onto grievances against others for many years. By the 1920s, he was also suspicious and conspiratorial, prone to believing that people were plotting against him and that there were vast international conspiracies behind acts of dissent. He never attended torture sessions or executions, although Service thought Stalin "derived deep satisfaction" from degrading and humiliating people and enjoyed keeping even close associates in a state of "unrelieved fear". Montefiore thought Stalin's brutality marked him out as a "natural extremist"; Service suggested he had tendencies toward a paranoid and sociopathic personality disorder. Other historians linked his brutality not to any personality trait, but to his unwavering commitment to the survival of the Soviet Union and the international Marxist–Leninist cause.