When Vos does not acknowledge her remorse for killing the butterfly, Girder’s smiles because her plan was successful.
Empathy had prevented Vos’ from taking full control over her hosts and she figured out a way put an end to that.
The one detail about Possessor most clear by the end is Tasya Vos’ descent into sociopathy, as identified by her sudden lack of remorse at the butterfly she killed as a child.
Yet, this evolution was no accident and much of the film’s central assignment was by Girder’s design in hopes to condition her “star performer” into an emotionless killer.