I never got that line. They are not really dead now, are they? So it should be more like: "Even after defeat, I can still be useful."
Well they're not saying "Even dead I still serve" - to me "in death" sounds more like being on the verge between life and death, the last stage before being dead. And that's basically what they are. The only thing keeping them alive is the sarcophagus, rendering them unable to die, thus - in my opinion - "in death" sounds fitting. I always thought it to be similar to "dying" - then again english is not my mother languague.