In the American cut of Metropolis the mentioning of Hel is completely removed from the movie. But it pretty much explains Dr. Rotwangs actions. Why was such an important story-element taken away?
I did a little bit of online research on this, and it would appear that the American distributors considered the movie too long, complicated and controversial and employed the playwright Channing Pollock to make a new version.
The American release was considerably shorter than the original (at 115 minutes it was about 25% shorter than the original) and massively changed the plot - to the point where some scenes became almost incomprehensible. According to one reference I found Hel was dropped because her name was too close to Hell - unbelievable really.
Pollock's version was released in the USA and the UK. Other adaptations of around that length or some even shorter (91 minutes) were released in the USA and Germany. The 91 minute version was the most commonly available from world-war-2.
I found a description about the restoration of the movie which contains some detail about the different versions.
This research has made me want to see the restored version, as I saw a version around 1992 with live piano accompaniment - and I now have no idea how butchered the version I saw was.