The only explanation is that the sequence was made first and the context was added later to justify Isaac moving from the Church to the Cultist Residence, ala some setpieces of the Uncharted series. Nothing explains why it was where it was, what it even is and why Isaac must fight it. It also, weirdly enough, has no context across the entire game. The Tormentor's chase sequence at the end of Chapter 5 is incredibly awesome to look at, is fun to play, and is one of the most memorable moments of the game.

A properly upgraded Contact Beam or Seeker Rifle can end the fight in as little as 10 seconds without the Necro reinforcements ever spawning. The whole thing can be over in minutes even on the highest difficulty. Despite waves of Necromorphs rushing at you, you can ignore them and just shoot the boss and they'll all die once you get to the next part of the fight. You essentially just have to shoot at the main target, then at the Marker. The Tormentor has an awesome design it's unfortunate that the 'fight' against it is more or less a quick-time event.(The fact that they are huge just makes it that much funnier.) Of course, most gamers will read something different into white stains under black lights.

Accidental Innuendo: In a later level of the game ( in which you revisit the Ishimura), there are ultraviolet floodlights set up to aid decontamination crews in scrubbing up bloodstains and such.

Dead space 2 chapter 13 laser room