9. John Connor - Terminator: Genisys
This entry is slightly different to the rest on the list, in that while the previous hero of the franchise made the change to evil in a sequel, it didn't actually take place on the same timeline. Terminator Genisys follows a completely different sequence of events from the movies that came before it.
For four previous movies, John Connor was the hero of the Terminator franchise, and his life needed to be protected. The fifth in the franchise however, followed a different timeline in which he was caught by Skynet.
The villainous corporation, personified in this case by Matt Smith, essentially killed John Connor's conscience and took over his body, sending humanity's greatest hero back in time as a T-3000. What better way to stop meddling heroes from getting in the way of an evil plan than by literally sending the best of them back in time to fight them?
After Terminator Genisys' underperformance both critically and commercially, Dark Fate followed several years later and instead of building on the story, essentially retconned everything. John Connor was killed, erasing Rise of the Machines, Salvation, and Genisys from the timeline. Still, 2014 had that brief window where humanity's greatest hero became it's strongest adversary.