A company known for ruthlessness has turned into an unexpected model for environmentalists by pushing to green its stores and its 100,000 suppliers. That effort intensified in July when Walmart announced plans for a sustainability index that will rate suppliers on such issues as energy use and labor, an area the retailer itself still struggles with. The goal is to improve efficiency by helping customers know at a glance how green a product is. "It's a way to minimize costs," says Matt Kistler, Walmart's sustainability chief. "It's also the right thing to do."