I think that the Baron and his keep simply served as a base where dead bodies pillaged from surrounding burial grounds by his brigands could be pooled and readied for Soulblighter. Soulbighter probably simply created that huge mass of thrall (50 thrall across and 100 deep was it?) when he visited and created the Stygs. After all, does the Baron... and helpless fat man with not a single evasive, offensive, or defensive spell of any kind, really seem like a necromancer who could created a legion of 500 thrall?
As for the reason behind the assassination of the Baron, he was the ring leader, the rich guy who was running the grave-robbing operation, without him I very much doubt the Brigands would continue their risky business. He was killed for the same reason the CIA strives to assassinate drug-lords and such.
-Milky
