Physics of the Crowbar

A parasite sprang from the rubble and he smashed it down viciously. The crowbar pinned it to the floor, its innards leaking even as it scrabbled for escape. The thing whined and screeched and died, fighting every second. Maybe a biologist could have told him why the thing was hostile, or what mechanism drove it to seek a human host, but he was not a biologist. He was a physicist, and he'd decided to live.

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Narrative of the first game.
Cross-posted from AO3; complete work, 32 chapters. Rating is for cannon-typical violence. No spoilers for later games.