Harry’s "Stop Voldemort’ plan was threefold: 1) Stop Tom Riddle from opening the Chamber of Secrets; 2) Stop Tom Riddle from becoming Lord Voldemort; and 3) Save everyone. Harry thought that objectives two and three could perhaps be collapsed, but he had to entertain the possibility that Riddle was clever enough to ruin the wizarding world in new and unique ways that didn’t involve becoming Lord Voldemort, like perhaps becoming Lord Mortevold.