In the 52nd century, (COMIC: The Master Plan) Krasko murdered around 2,000 people and was imprisoned in the Stormcage Containment Facility, (TV: Rosa) his period of imprisonment overlapping with that of the Master. (COMIC: The Master Plan) He was subject to rehabilitation and had a neural restrictor installed into his brain before he was released, preventing him from inflicting harm or killing any living thing regardless of how much he wanted to. (TV: Rosa)
Krasko believed that black people got "above themselves" and identified the Montgomery Bus Boycott as when "things started to go wrong". He used a vortex manipulator to travel back to Alabama 1955, taking with him time displacement equipment and intending to prevent Rosa Parks from refusing to give up her seat on 1 September by having James Blake replaced by Elias Griffin Jr and limiting the number of passengers.
After detecting and identifying the TARDIS, Krasko found the Thirteenth Doctor, Yasmin Khan, Ryan Sinclair and Graham O'Brien with his equipment and threatened to kill them if they did not leave Montgomery. They did not do so and, when Krasko blocked the bus's route with a car, Ryan used his time displacement weapon on him, sending him back in time, (TV: Rosa) likely to the time of the dinosaurs. (WC: Case File Three)
Krasko's racism led him to believe that "things started to go wrong" with Rosa Parks' victory. He told Ryan that "[his] kind would be put in [their] place". He was sadistic as he showed no remorse for the two thousand people he had murdered and when recalling his crimes to the Doctor, he smiled with pleasure. He also made a joke about not really remembering how many people he had murdered.
Because of the restrictor in his brain stopping him from inflicting pain or killing other people, Krasko was visibly frustrated when the Doctor snatched the vortex manipulator from his wrist and stomped on it, not being able to strangle her when he managed to overpower her briefly. He also didn't care about the Laws of Time and what could happen if Rosa Parks did not make her historic victory. (TV: Rosa)