Assassinations have the chance to happen randomly. Solterra became rich, he could have just called for the best spy in Lundlands to assassinate Lord Wolff or Lord Cole, but he didn't. Nor will he still. Solterra isn't an asshole, but his attempts at being good always give the bad impression.

The assassination could have even happened by someone else that had a dislike for him. I am happy to know the Vassals thoughts are under Flooby's rule, because the fact is that Solterra didn't know when or why the soldiers would be raised against him unless he had spies already inside and gaining information, so that means he couldn't have planned it as you say.

Cole was a man of trust to Solterra, he trusted him to protect his sister and was to be forever in his debt to the service he offered. What kind of a man puts his greatest trust in someone and then holds paranoia that he may plan against him? It may have been Solterra's flaw, but at least he acted honorably and respectfully.