An Evil Guest -- Never Set Yourself Up for Failure
"Never set yourself up for failure. Never," is the motto of Gideon Chase. He aims for an ambassadorship to Woldercan, and he gets it. Therefore we can project that all the events in the book are basically as he planned and anticipated. He saddled his enemy, Bill Reis, with a weakness in the form of a love for Cassie Casey, and got her out to Cthulhu territory. Bill's attempts to protect her and retaliate for his enemies' attacks on her lead to his death. So insofar as this story has Cassie Casey as a heroine, Gideon Chase is without a doubt the villain of the piece.
By the end, Cassie has taken two guns with her to Woldercan, and manages to steal a very long-range spaceship. It's very possible that she killed Gideon Chase in the process. For more of this theory, see the Mysterious Margaret thread.