The lizardfolk shaman is a CR2 monster with druid spells. It has this ability:
Change Shape (Recharges after a Short or Long Rest). The lizardfolk magically polymorphs into a crocodile, remaining in that form for up to 1 hour. It can revert to its true form as a bonus action. Its statistics, other than its size, are the same in each form. Any equipment it is wearing or carrying isn't transformed. It reverts to its true form if it dies.
If the lizardfolk shaman is killed while in crocodile form, then it reverts to its true form.
Is the true form then at 0 hp? Or is the true form at however many hit points the creature had before it used the ability?
I am guessing that the ability is intended to be similar to the druid's Wild Shape feature. But here's the text of the werewolf's Shapechanger trait:
Shapechanger. The werewolf can use its action to polymorph into a wolf-humanoid hybrid or into a wolf, or back into its true form, which is humanoid. Its statistics, other than its AC, are the same in each form. Any equipment it is wearing or carrying isn't transformed. It reverts to its true form if it dies.
...And I think the werewolf actually stays dead if killed in its alternate form.
What happens in the case of the lizardfolk shaman?