I am thinking they needed to shut the gas off before the freighter folks came on island to kill everyone so that Ben could not gas the island and protect his folks..

Maybe the Temple is a type of "fallout Shelter" like a bomb shelter, that way when Ben if he needed to use the gas they would still be protected?

__________________
Please don't give up, Des. Because all we really need to survive is one person who truly loves us. And you have her. I will wait for
you. Always. I love you, Pen

DESMOND: I love you, Penny. I've always loved you. I'm so sorry. I love you!
PENNY: I love you too.
DESMOND: I don't know where I am, but--
PENNY: I'll find you, Des--
DESMOND: --I promise--
PENNY: --no matter what--
DESMOND: --I'll come back to you--
PENNY: --I won't give up--
BOTH: I promise. I love you.