Seem the monster shows you your inside. Locke saw a beautiful light while Eko saw a horrifying black cloud,
Ok, didn't the others try to capture Ecko because he was good?
Also, the thing hunted Jack and company a few times, so Locke is the only good one?
EDIT: Also, when Ecko leaves the beach, how can Locke not track him in SAND!
EDIT: The new couple is like a Boone and shannon replacement