But before Maul can kill him, Obi-Wan pulls some elaborate Jedi shit and cuts the Sith Lord in two as he, not Obi, falls into the pit.
Why it's baffling:
It's like at the last minute Maul completely lost his mind, or maybe had some kind of seizure. After managing to handle both Jedi at the same time without even a scratch, and getting Obi-Wan into that precarious position over the pit, what does he do?
He stands at the edge and starts mindlessly slashing his blade at the floor, like a kid who just got his first lightsaber. We're not kidding. This happens. He seriously just starts slapping the floor with it, to make sparks shoot up.
So he winds up standing there on the lip of the chasm for a long time. This whole time, he has Obi-Wan at his mercy. He knows Obi-Wan is a Jedi and thus can do some superhuman somersaulting tricks, and to do him in all he needs to do is lean down and knock Obi-Wan off his perch with his lightsaber. Really, just kind of reach down and poke his hands with it. Or just pick up something nearby and drop it on his fingers.
Instead, he just stands up there while Obi-Wan sloooowly looks over and notices Jinn's lightsaber lying on the floor, then slooooooowly figures out what he's going to do. Then he ponders it for a few seconds more. Then, finally, when Obi-Wan begins to make his move, this confused looked glazes over Maul's face. What did he think was going to happen?
Obi-Wan somersaults over Maul's head, retrieves the lightsaber and then lands behind the Sith. Maul has time to turn and look, standing perfectly still for a full second ...