Well, strictly speaking, he already knows he didn't, because they had a conversation at the end of The Soul Key
before he left. So he doesn't really think he killed her.
His announcement here that he killed her was partly to truly confess the depth of his sin and express his guilt by casting it in the worst possible light (after all, she was technically dead on the slab for a brief period in Warpath
, or rather in episode 9x18 "Renegade"), and partly so that I could leave that storyline on a bit of a cliffhanger with the question of whether Odo would be so enraged as to actually kill him.
And the season's actually only going to be 22 episodes. That's more the standard season length for most American network dramas, Trek
was unusual in going for 26. And in fact Enterprise
was cutting its seasons down to 22 by the end too, so there is precedent.
But thanks for still reading! Lots more to come.