Sayid kills Dogen & Lennon clearing the way for Smokey

At the temple, many of the others are scrambling to gather their belongings to join Locke before sundown. Lennon tries to tell them that they're all safe as long as they stay at the temple, but even Cindy is not buying it.

Sayid goes inside to the spring, where Dogen is sitting on the steps holding the baseball. Sayid sits the dagger on the ground and Dogen says that Sayid let Locke talk to him (so, it would have actually worked if Locke hadn't spoken before being stabbed?).

Dogen tells Sayid that he was once a successful  business man in Osaka but after drinking to much at a celebration for a promotion, got into a wreck after picking his 12-year-old son up from baseball practice (that was likely his son we saw in the flash-sideways in "Lighthouse"). His son was badly hurt or possibly dead when Jacob approached him at the hospital and offered to heal his son, but in return, Dogen would have to come to the island and never see his son again (sounds a little like Juliet's story, minus the personal contact with Jacob).

Dogen then asks Sayid if he will stay or go and after Sayid seemingly sincerely says that he'll stay, he grabs Dogen and drowns him in the spring. Dogen releases the baseball in the water as he dies. Lennon then comes running in, berating Sayid for what he'd done. Lennon says that Dogen was the only thing keeping "him" (Locke) out. When Lennon is distracted by the warning sound of the smoke monster, Sayid slits his throat with the dagger and his body falls in the spring. Sayid has clearly gone dark.

So how exactly was Dogen the only thing keeping Locke out of the temple? Is he somehow responsible for the ash? If Locke's team really is the evil side, Sayid has very clearly joined it in a very dark way. He seems a bit like Michael in season 2. Sayid is willing to do anything, even murder, to get Nadia back.

Jacob appears at the temple + Hurley & Miles play tic-tac-toe

At the temple, Hurley and Miles are playing a game of tic-tac-toe on the ground with sticks, leaves and vines. When Hurley goes to the spring to try to find some food, he sees Jacob on the spring's steps, staring at the water.

Jacob tells Hurley that someone is coming to the island and that Hurley needs to help them find it. He says that he'll need to write a few things down.

The water in temple spring is brown, Sayid dies after dunking

Dogen leads the surviors through the temple where we can see a few natives/others working the the grounds. They are then led to the interior where the spring is located. Hieroglyphics are visible on the pillars of the room.

As they enter, Lennon notes that the water isn't clear and asks what happened. Dogen says that he doesn't know and proceeds to the steps of the spring, cuts his hand with a knife and submerges it in the water for a few seconds. He pulls it out and it hasn't healed. He then tells them that there will be risks if they heal Sayid and Jack, continuing his new trust fate kick, immediately agrees that they should proceed.

A few of the men remove Sayid's coveralls and proceed to carry him into the spring. They submerge him once Dogen has started an hourglass which appears to be filled with a mix of dark and light sand. Sayid starts kicking fairly quickly but they keep him down until the hourglass runs out and he is no longer moving. Sayid is in a Christ-like pose as he is carried from the spring and then Dogen confirms that he is dead.

Jack then briefly tries to resuscitate Sayid (which is similar to when he saved Charlie after he was hung  by Ethan), but gives up at Kate's insistence.

So was the water not clear because Jacob is dead? What exactly is the spring and how does it work? Presumably, this is where Richard brought Ben after Sayid shot him.