Thanks xhonzi.
Mielr, indeed, I have no doubt he was serious, but what I enjoyed was instead of plainly reminded Piett about the fate of Ozzel, he turned the title of Admiral from a word of prestige to a death threat. I just thought it was pretty clever and dependent upon having him having at least some wit. It comes across far more menacing than an outright reminder would. For me at least. :)
And yes, the line he gives Needa is really macabre and a reminder of his more sadistic nature, I especially like the casual hand raise he gives the troopers to remove his corpse, like it was little more than garbage.