system
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human
[40K] How would a space marine act should he be captured by xenos? How would a guardsman? Details included.

More specifically, if they were "kidnapped by aliens". They are held for study/interrogation and are unable to break out and escape. What is their attitude towards their captors once they realise that there is no hope of returning to Imperial Space? There's no torture involved, merely coercion along the lines of "let's be friends, we want no war, please help us attain peace and understanding" type deal.
gpt
A guardsman would be fishpaste if caught by *most* xeno races.  The Dark Eldar would sell him into slavery, although he still might get killed first.  Only the Tau would recognize something like our contemporary rules of war, and would offer the gue'la freedom - on Tau terms - or would perform a prisoner trade.  (But any guardsman who was traded back to the Imperium would be interrogated and summarily executed by the Inquisition, so he'd *still* be fishpaste.)  For as long as they were permitted to act, they'd perform the duties in their *Imperial Infantryman's Uplifting Primer*, which instructs them to go down killing before they killed you.

Oddly enough, the space marine battle strategy is similar - except *their* goal is different:  each space marine is host to an incredibly valuable geneseed, the source of their superpowers.  If the geneseed is lost, it represents a heinous blow to the imperium; but if it falls into enemy hands, it represents terrible danger.  So, they fight with two goals:

1)   Get back to Empire-controlled territory, killing as many xenos as possible on the way out.  
2)   If 1 is not possible, ensure their body is completely annihilated - preferrably in an action that strikes a blow against the enemy. 

Some enemies would act differently.  Chaos followers of Nurgle and Slaanesh would relish the opportunity to convert an extant Space Marine; Khornites woud kill him outright; and Tzeentch would vary on whether it suited its plans or not.

Individual chapters would fight differently.  Ultramarines have the not-quite-heretical belief that "The Emperor provides," and have a grim respect for the Tau; recalling the times that the Ultramarines have shown the Tau mercy - and the times they have worked together, fighting Chaos on Dolumar IV, Necrons on Malbede, and the numerous times they have fought together against the Tyranids, an Ultramarine might be able to parley with a Tau captor.  Grey Knights, dedicated to fighting Chaos, might work with the Eldar - the only race which is as concerned with Chaos as humanity is - although the chainsword would eventually bite into an Eldar neck, as the Eldar have fallen before.