What happens if Humble Defector is sacrificed before its ability resolves?
Yes. Abilities exist on the stack independently of their source.


  112.7a Once activated or triggered, an ability exists on the stack independently of its source. Destruction or removal of the source after that time won’t affect the ability. [...]


That means that even if the object that has the ability changes or ceases to exist, the ability spawned from it will go on to resolve as normal.

In this case, that means you get to draw two cards, then the targeted opponent attempts to gain control of Humble Defector and fails to do so.