The reason why Su Mo wanted to kill Wu Ma and Wu You Lan wasn't because of their storage rings, but for their special body constitutions. Wu Ma had a Coloured Glaze Treasure Body while Wu You Lan possessed an Elemental Spirit Body. Although neither was an extraordinary body constitution, they were both ranked in the 700s on the Archean Special Bodies list.
Hence, Su Mo was willing to take the risk of killing the both of them. Once he devoured their lifeblood, he would possess their special body constitutions. He would then possess 4 special body constitutions and his talents would increase drastically.