Yes, the butler is a traitor. It's a cliche, but it fit the narrative very well, so I went ahead and used it. I don't think a lot of people will see it coming, though there are several clues. One of them is the fact that he makes only one cup of tea here and brings it for Wax; Tillaume is not accustomed to killing people, and he's extremely nervous in this scene. That's why he made the mistake of not making three cups and bringing them all over. (My writing group caught this, which amused me. They thought it was a mistake in the writing, though.)
One of the things that made me want to write this story, and keep going on it after I'd started, was the chance for good banter between Wax and Wayne. They play off one another well, and I haven't had a chance to do a book in a while (ever since the first Mistborn book, really) that had a good, long-established relationship between main characters who I could play off each other in this way. There is something deeply satisfying for me about this kind of writing, even though it's really just silly banter. I feel as proud of moments like Wayne toppling over because of the tea, then the conversation in the speed bubble, as I do of a deep character complexly coming to a character climax at the height of a story. That's because, at least as I see it, this is as technically difficult to pull off—the right feel of two characters with a very long relationship, talking in a way that conveys their years of experience with one another. And, at the same time, hopefully being amusing and interesting.
It's very dense writing, for all the fact that it doesn't read that way. (Unlike, for example, a really good section of dense description, laden with meaning.) Part of the reason it works is because it feels so easy to the reader.