@alidan
The mc does not need to kill innocent people to end slavery, it is all about the story telling. The author can have the mc pick his targets to avoid risk to the population, and how he achieves that can be as creative as the authors imagination. Also you don't need a powerful mc, since...
@alidan, your assumption that he can't do anything about slavery is false, since most mc are overpowered and can do anything they want but choose the most boring and simplistic route because the author does not know how to write a good story