Have you seen an excerpt from the Williams interview? I don't know if all interrogators are the same, but it was clear from the interview that Smyth was not trying to prove Williams guilty. In fact if anything, he was trying to help Williams by providing DNA and a boot print to help direct suspicion away from him. It's not Smyth's fault that it had the opposite effect. He even asked Williams to please explain these things away, hoping that Williams could put some doubts in his mind. Instead, Wiliams could only sit there and say that he didn't know what to say. Well, again, not Smyth's fault.
While none of us can get into Mr Smyth's head, I don't believe for a minute that he was on William's side. Interviewers use different tactics depending on the suspect. Interrogation conjours up all sorts of images from the dark room with a single bright light, sleep deprivation, waterboarding etc. While brow beating might work on a nervous street punk, a decorated senior Air Force officer would likely become defensive if aggressive tactics were used. The gaining of a suspect's trust, having him or her think you are on their side does work wonders to break down their defenses. A belief that honesty will garner sympathy is the prosecution's best ally. Just think of that friendly, conscientious police officer who last stopped you for speeding, "Good day, (sir of ma'am), how are you today? Do you know why I stopped you? Do you know how fast you were going?" How many would answer, "I don't know"? Or "Oh, a hundred and two"? Gotcha, you've either admitted to speeding or qualified the officer's evidence against you.
Here we have an experienced and talented interviewer who used a time tested tactic with a desired result, and he did it with panache, I would have had to get out of the room a few times to punch a couple of walls, and then shower with Javex by the time it was over.