Yesterday I kicked off running a Deathwatch campaign with two friends of mine acting as players. I started out with running the Extraction adventure from the back of the Deathwatch core rulebook., and the two man kill-team consisted of a Librarian from the Ultra-Marines chapter and a Tactical Marine from the Dark Angels Chapter. To my surprise, my two players immediately concluded that saving Magos Vyakai meant potentially leaving every other individual still at the refinery to die.
During the firefight at Location 6, the tactical marine concealed himself behind some cover and began sniping the Tyranid warriors with his stalker pattern bolter, and the librarian attempted using an incendiary grenade on the horde of hormagants before discovering his smite power did tons better when reducing the horde magnitude. The librarian managed to take out both Tyranid Warrior when activated his Might of the Ancients power and waded into melee combat with them.
After the fire fight, the tactical marine player decided he took exception to a commissar shooting one of his own men. The marine showed his disdain by taking a full action to aim and then using his next action to snipe the commissar with his stalker pattern bolter. Given my low to moderate knowledge of the 40K fluff, I may have decided wrong when I concluded the remaining Imperial Guard soldiers in the commissar's unit took no offense of a space marine executing their superior officer.
They sent the guardsmen to patrol another section of the refinery, and the kill-team found the magos in the next place they searched. Instead of waiting for the guardsmen to return, the kill-team called for an extraction by thuderhawk gunship at the first opportunity.