Very frustrating indeed. I remember thinking, this guy is incredibly sane for a mid-thirties, very active channeler. It occurred to me that he was one of the forsaken, given the book theme at that point. He certainly had some kind of protection from the taint. I eventually tossed that idea when he agreed to be a school teacher. On top of that, he himself mentioned he had been channeling for 15 years. Only Ishy was around and he was definitely dead and quite mad. Darkfriend then.
Now, as to why Rand did not realise that, still bothers me. Too focused on Sammael in the beginning and then always something else. He also desperately needed someone to train these men, and Taim was the only one who could do it. So, maybe just blinded by his own need for his plans to succeed. He is a super Ta'veren afterall.
To be fair, Logain seemed relatively sane as well. But our only interaction with him before he was gentled is his insane laughter at seeing Rand. So maybe not completely sane afterall. But certainly seemed so after being healed by Nynaeve. Completely immune in fact. Maybe he hadn't been channeling for long before the reds nabbed him.
The whole thing just feels unpolished.