So basically, your question is "when Misa was captured, why didn't they just slaughter everyone instead of Light doing his gambit?"

Hm. I can think of a couple of justifiable reasons.

1. Shinigami don't seem to have much in the way of initiative. Rem may be in love with Misa, but he's still pretty passive. (Hence, he wouldn't think of this himself- he's relying on a plan from Light)

2. This wouldn't absolve them from suspicion- it would actually increase suspicion. That is, say they're investigating Misa, and suddenly the entire team is slaughtered. All it would take is for a "normal" policeman to somehow get the team's notes, go "Oh, Kira killed everyone while they investigated Misa," and that would be a huge warning sign.

3. Misa was being held captive and tortured- it's a pretty legitimate interpretation, I think, to say that Rem saving her would be considered "extending her life."


So, three possible reasons I can think of.