That the system give him a mission to kill Danzo, and that the reward is that he obtains the uchiha bloodline or that when he kills him, he gives him the sharingan.
If MC is going to ditch Konoha anyway, won’t leaving Danzo alive allow him to continuously sabotage the village for a long time? It’ll never truly grow past a certain point. Helps suppress a potential problem over the long run, no?