Why are the Jewish concepts - of what often is translated as - 'Hell' and which is often seen as a place for cleansing, purification, refining and correcting the soul (purgatory), related and connected to such a horrible place where some people used to sacrifice children)? How does the latter reflect what has become the first?
It seems you're alluding to the discussion on this answer and the comments there. The Midrash quotes in Fred's comment there explains that fathers would place their children in fires as offerings for the idol Molech, and the children's screams would reverberate from the fire (shekolo shel tinok nohem min ha'eish). Alternatively, the priests would chant incantations (nohamim) during this process. Therefore this valley was called Ben Hinnom, and it was in this valley that an entrance to Hell can be found (Eruvin 19a).