I guess the key point was that thet won't rape/butcher
unless ordered to. Dany starting the massacre was Greyworm's go ahead and then the northerners joined in.
The scene was definitely much more effective with the perpetrators being the supposed good guys. What with the WWs being defeated early in the season I guess there's supposed to be a "people are each other's own worst enemy" theme to the second half of the season and this certainly drilled that home.
It's a shame a lot of people have a problem with the Dany heel turn because the scenes of carnage that followed were really very well done.