This fast song brings the tempo back up again after the quiet ending of 'Blood Brothers'. There is not much to say about the music, except that it's a traditional Maiden fast track.
According to Bruce, the song is basically about a bounty hunter, but they apparently didn't want to call it 'The Bounty Hunter', probably because 'The Mercenary' sounded better as a title for this song. The lyric are in the same vein as those of 'The Assassin', about somebody who hunts and kills for a living.
It was brought to my attention that the lyrics were maybe based on the 1987 film Predator, with Arnold Schwarzenegger. In this film, a creature called "Diablo" by the natives is hunting down humans in some South American jungle to "make trophies out out of men". All this seems to make sense, but as none of the band members has mentioned this in any interview I've read, I doubt that the film actually inspired the song.