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.