You choose the target artifact when Werewolf Ransacker's first triggered ability goes on the stack. You choose whether or not to destroy the artifact when that ability resolves.
An artifact token that's destroyed is put into its owner's graveyard before it ceases to exist. If a token is destroyed by Werewolf Ransacker ability, Werewolf Ransacker deals damage to that token's controller.
If something becomes a copy of Werewolf Ransacker, that doesn't count as "transforming into Werewolf Ransacker." The first triggered ability of the new Werewolf Ransacker doesn't trigger.
If the targeted artifact has indestructible or regenerates (or you choose not to destroy it), Werewolf Ransacker doesn't deal damage to that artifact's controller. Similarly, if the targeted artifact is destroyed but a replacement effect moves it to a different zone instead of its owner's graveyard, Werewolf Ransacker doesn't deal damage to that artifact's controller.
For more information on double-faced cards, see the Shadows over Innistrad mechanics article (http://magic.wizards.com/en/articles/archive/feature/shadows-over-innistrad-mechanics).