Fridge Brilliance: The nameless protagonist of Doom 3 is probably actually called "Marine Transfer", considering that is what appears on the passenger list at the start of the game (alongside Elliott Swann and John Campbell).
Classic Doom, the Arachnotron and Spider Mastermind have, compared to other monsters, a disproportionately high chance of flinching when they're hit and being interrupted from shooting back. They're giant brains on mechanical legs. Of course they're going to be less resistant to pain.
Which has a bit of fridge logic in itself, since a brain is the ONLY part of a body that cannot feel pain at all (it processes pain but actual harm done to the brain itself can't be felt)
Possibly it's not actually pain, but unintended spasms caused by the punishment being rained upon them, then?
Fridge Logic: Lampshaded in the Doom II ending: the Marine wonders where all the bad people go, now that he just blew the fuck out of Hell.
The presence of chainsaws on Mars is also lampshaded in Doom III, where several PDAs complain about chainsaws ending up on a decidedly tree-less Mars. (They were a UAC shipping error.)
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