I guess it's because he's a prince from another country, so diplomatic reasons. And her mother's previous reaction to him purging the harem as he said to the pinky.
Maybe something related to his mother ... like promise not to kill .. them .. bla bla .. ...???
Cause they can just hide the dead body and pretend nothing happened
Dead concubine will require explanation
And Shuraka mentioned that it's cause of his mother, potentially the guy is from the same country
And Shuraka does seem to be sentimental about his mom
pink hair was a concubine from shurakas father. in chap 20 we see shurakas mom cursing shuraka for killing the concubines she was friends with. he's only keeping him alive for his mothers sake.
Being a concubine is also a rank i think. Its an official title in several historial empires.
In this story, he didnt kill the concubines bc of his promise to his mom.
why kill the guy not the pink hair one? like fr bro