It's etymology is given as:
hell-bent, 1835, U.S., originally slang, from hell + bent
How do the the words "hell + bent," when taken together, form the definition "determined to achieve something at all costs?"
Edit (thanks to Medica's response):
In this context, could somehow "hell" imply going to disproportionate or irrational lengths to achieve an end? The term "selling one's soul to the devil" comes to mind as well in this sense.