You could easily stick with the Nurgle theme idea, considering Nagash merely pushed Kemmler in the right direction, so to speak. And I doubt that Nagash would really care if/when Krell receives some gifts from Nurgle.
how far fetched would it be to write fluff along the lines of Nurgle taking direct influence, through Nagash, bumping off Kemmler - Krell is then granted an army of the dead ie. risen WoC with the mark of Nurgle risen bu Nurgle using his Undead Plague, and Nagash, seeing the power granted to Krell, uses Krell and his army of dead Chaos Warriors and obvious favor of the gods as a tool to gain back the magic artifacts he has lost and is in need of to become as powerful as he once was (such as his crown locked in the Imperial vaults, his talisman from Mannfred von Carstein, etc.)
Obviously with more details and storyline added, but put simply, Nurgle gifts Kemmler and Krell with his mark and other "gifts", as well as an army of Chaos Warriors brought back from the dead using the Undead Plague. Kemmler is driven mad by the gifts and influence of Chaos, but Krell retains his existence and general consciousness, filled with hate (being a favored champion of Khorne in the past) and armed with an army of undead Chaos warriors/broken free of his bonds to Kemmler, he rampages across the countryside so to speak and unbeknownst to him, he is being manipulated not only by Nurgle (who wants his pestilence spread far and wide) but also by Nagash, who wants his artifacts to regain power.
EDIT: As a side note, this is getting pretty close to belonging in the "Homebrew" section, so if it drifts too far in that direction I apologize. Its kind of a grey area, as it is cool to learn more actual fluff on these guys (Krell in particular) and I want to make sure the ideas I have fit with existing fluff. I will write out a homebrew thread when I get the actual fluff for this army written, but first I need to learn all I can of the existing fluff to lay the ground work for my homebrew army