This is a stab in the dark but Jigen could have stolen Sasuke's power and given to Kawaki via karma seal. Sasuke's power came from his experiences as a child that motivated him to seek the destruction of the shinobi world. Kawaki seems to want the same thing post timeskip. He also had a bad childhood just like Sasuke so if he were given the same power of darkness it could easily corrupt him and cause him to turn on the village. It would also add an interesting layer to the Boruto vs Kawaki fight if Boruto would have to defeat his rival/sensei to save the village.