We also don’t know if Snow’s heart is actually turning dark, or if Regina was able to make it appear as if it were. Regina could have realized in that moment that the potential for Snow to “turn on herself” would do more damage than Regina ever could. So she could have decided to help that process along by showing her a “darkening” heart.
Highly doubt that. Seeing how much Snow already has been beating herself up about what she did before, there is no need for Regina (or the writers) to further that with a faked dark spot on Snow’s heart. Regina could have just let her go without ripping her heart and showing that spot and Snow still would be beating herself up about what she did and ravel in self-doubt more and more.