Thom has actively and directly opposed the shadow too many times in the series to be a darkfriend. While he doesn't drive the plot along himself (mostly) he's been crucial to Elayne, Nynaeve, Rand and Mat many times, and could easily have thrown a spanner in their plans if he'd been so inclined.

He 'knows a lot' because that's his function in the series. Moiraine is the wise-but-unapproachable Gandalf figure; Thom is more human, and his knowledge more down-to-earth. They complement each other. And he murders Galldrien - not a darkfriend - in a cold rage, after he finds the woman he loves dead. He's only human, and if his conversation with Moiraine later on is any guide, is not averse to revenge.

(In that vein - can you imagine what Perrin would do if someone killed Faile? Bloody dismemberment would be getting off lightly. And he's as lightside as the come.)