I thought Teal'c had just snapped as well (before I saw Threshold s5e2) but it didn't seem to be in his character. That's what really had me thinking. Threshold showed him in his early years up to becoming the First Prime of Apophis (replacing Bra'tac). Teal'c frowned on the practice of killing innocents and saved a fellow warrior destined to be killed by Apophis. When he saw that Apophis did not detect his disloyalty, he knew it was time to act. Hence, his switching of sides in the Stargate SG1 pilot.
Regarding seasons 9 and 10, I've only seen a few episodes from those seasons (I didn't want to get into the later seasons, wanted to start at the beginning) but they don't compare to the early ones I've seen so far. I wouldn't call them crummy or anything though. More like they have a slightly different feel to them.