Socialism is not extreme.
you quote Stalin,
he was not a socialist
he was a Georgian revolutionary and Soviet politician who led the Soviet Union from the mid-1920s until 1953 as the general secretary of the Communist Party of the Soviet Union (1922–1952)
Whether he practised communism or state fascism is debatable ,but he was not a socialist.
it is arguable but some would argue that the USA behaves and has behaved in a terrorist manner, the overthrow of Allendes government was a terrorist and extreme act of violence, it wass extremist to overthrow Allende who was democratically elected.