Apparently Bruce Dickinson wrote it for the soundtrack (it is listed in the end credits), his version is less agressive; Steve Harris heard it and thought it would make a great Iron Maiden tune so they brought it out as a single. I ended up using Iron Maiden's version as I think the playing on it is much tighter.
Always been a favorite of mine as well, I can't remember if I first heard it on the album or as a single.
Here is the original version (which won a razzie award for worst song of that year):