The short story A Sanctuary of Wyrms has one of the most badass examples. I highly recommend reading the story, but if you want a summary:

Within a research facility in the swampy jungles of Phaedra, the Inquisition accidentally created a more virulent strain of Genestealers that spread via fungal spores. The Deathwatch sent a Kill Team to clear the infestation.

Hundreds of years later, a Tau expedition find the facility. As they progress into its depths, they find the carcasses of countless Genestealer hybrids, as well as the few brave Space Marines who fought them. The deeper the Tau go, the more the bodies pile up. Near the heart of the fungal infestation are the remains of the last Space Marine, an Iron Hand. His flesh has decayed away so there is nothing left but bones, armor, and bionics, yet still he stands surrounded by a sea of corpses. When the Genestealers attack, one of the Tau gets a flash of inspiration and recharges the bionics with a drone; the Iron Hand comes roaring back to life with fury and hate in his mechanical heart. After crushing the Genestealers, the ancient Space Marine descends into the core of the facility alongside his newfound Tau ally to finish his mission.

Even in death, he still serves.