The tower magician harness Rood's brother as source of magic, and that source of magic is running out soon. They just want to replace the used up source of energy to another, which is Rood.
I'm more towards Lispen being the killer of Kielnode, attempting to break Rood and made it seems like it is the human that kills Kielnode. Rood will embrace his everything in despair and possibly be vengeful towards Human, this works very well for Lispen.
Hence he smiles