In literature as in life, it depends entirely on why you did it. Sadism implies enjoying pain for its own sake. Literature that does that is no less sadist than worlds that do. But literature that includes pain is not generally evidence of sadism, and neither are worlds. What you can say about the creator's character is entirely driven by what you can say about his purpose.

A simple motivating example: I spent a while undergoing some martial arts training, and at times it was painful. Injuries weren't intended, but could the class have possibly been conducted with a little more deference to my sore feet, with perhaps one less bruise here or there?

In a philosophical sense, of course it could have. But this is not evidence of sadism on the part of the instructor (or the fellow students who gave me such a beating). In a practical sense, spending too much effort avoiding pain would have undermined one of the purposes of the class: teaching me to fight through it. Teaching me that some things are worth enduring pain to learn.