Is it a sin to kill or?

Ok, imagine that you were sitting in your house peaceful and a woman knocks your door. She tells you that her kid is drowning and beggs you to save it. What will you do? Save it offcourse. If you let it die it will be a sin. So! After many many years you find out that the kid you saved is a very bad person for example HITLER. You have saved a kid that killed millions of people. That means YOU killed those people by saving the kid! Isn’t that a sin too?? If you were in a situation like this what would you do? SAVE OR LET THE KID DIE??