Well since walt is now a cryborg child after the others injected him and changed him... we dont need to worry about walt.

Besides... Michael is that type of person. you can take the crazy violent guy off the island, but you cant take the crazy voilence out of him.

i forsee future violence by the hand of Michael that could physcially affect walt. so we should eliminate the threat... or let sawyer or hurley do it for us.