Two things:
ONE:
Killing an innocent child? ARE YOU CRAZY? Yess, okay, people wouldn't die from cancer etc., but wouldn't you feel guilty about something which hasn't yet been corrupted by all the shit in the world? A child has the potential to grow into someone
amazing, and also, WHAT IF THE CHILD WAS YOURS?? Would you still kill he/she to save others? Yes, I may be selfish and a crazy cat lady etc., but sometimes diseases are there to do bring out the best in people. If there were no diseases there would be no doctors, pharamacists etc., so life wouldn't be worth living for some.
Also, the death of the child means that there would be unnecessary sadness and pain in the world. There is nothing worse than being told that someone you love has died.

TWO:
Hitler: OUCH! Although he wasn't the most, well, great person alive, at least in retrospective we learnt a lesson from him, right? And also, we can't be too mean on him right? Yes, millions died because of him( and that was really horrible), but he was still a human, and we should treat him like one.
s.