Thufir Hawat is the Mentat Master of Assassins in service to Duke Leto, and he's one of Paul's three teachers. Before we dive into Hawat's character and analyze him for all he's worth, perhaps we'd better figure out what's up with this whole Mentat thing.
And now for something completely different: fictional universe history. It's just like actual history, only with ray guns.
In the Dune universe, a war occurred thousands of years ago called the Butlerian Jihad. To make a long story short, Serena Butler lead humanity in a revolt against their machine masters. These machines, called Thinking Machines, were ultimately destroyed and cast out of human culture. As the Reverend Mother quotes from the Orange Catholic Bible: "Thou shalt not make a machine to counterfeit a human mind" (1.140). The result was that humanity could no longer rely on computers to do logic computations or run space ships or add 2 and 2. Yep, the abacus makes its long overdue comeback.
(Psst: you can read all about the Butlerian Jihad in the Legends of Dune trilogy, written by Brian Herbert and Kevin J. Anderson.)
Humans then trained themselves to do the jobs of the Thinking Machines. The Spacing Guild used spice to learn to fly ships through deep space, and Mentats developed extensive brainpower and memory retention. Using huge stores of mental data, they advised the leaders of the Great Houses on issues ranging from politics to military campaigns to fall fashions.
What does all this have to do with Thufir Hawat? Well, this…
In the Dune universe, Hawat's logical capabilities are supposed to be second to none. We say supposed to be because, um, he certainly doesn't seem to be all that logical at times. In fact, the reader will often be screaming at Hawat when every move he makes seems completely illogical. Ah, dramatic irony, how frustrating you can be.
The character of Hawat is a basket case—er, a case study on logic and reasoning. We're talking about logic and reasoning not as theoretical sources of knowledge, mind you, but as corruptible one. Why corruptible? The problem doesn't lie in logic and reasoning alone but in humanity's imperfections. If we are corruptible, then we can corrupt any source of information and knowledge given to us, even the good stuff we receive through logic and reasoning.
For Hawat, this corruption of knowledge comes from two things. The first of these is pride. Hawat's pride in his abilities blinds him to the possibility of Yueh being the traitor. Jessica points this out to him, saying "it's a human trait that we encounter personal problems, those things most deeply personal are the most difficult to bring out for our logic to scan" (17.147). But Hawat's pride just writes this off as Jessica hoping to "undermine [his] faith in [his] abilities as a Mentat" (17.148).
The second corruption comes from false information that Hawat interprets at face value. Hawat believes Jessica to be the traitor, so the Baron Harkonnen feeds him false information to prove just that. With his pride on the line, Hawat swears vengeance on Jessica. This vengeance almost leads him to take Jessica's life and ultimately does lead him to design the plan that kills Paul's child.
It's all because of a little misinformation and a man's pride preventing the proper analysis of information. Wow, this book can get kind of depressing, can't it?