How did stalin (ussr) stay president if he was so cruel to his people?

Because the presidency was decided by a vote he simply rigged the vote. that and he had followers i mean one man against a whole country that doesn't like him doesn't seem to hard to pick the wjnner

Because Russia (USSR) practice communism in which means no people right to vote for the new leader...back to the question and Stalin has secret police which kill people who try to oppose Stalin leadership...therefore he also carried out show trials, Assasination of party members and massacre of innocent people in Russiaeventhough no one dare to oppose him during his leadership