I mean the bar was already set to a bullshit level.
MC already spared Shirushiru, someone who willingly killed a ton of people for funsies, just because she begged for Mirumiru's life instead of her own.
Honestly I'm kinda hoping MC's sister's plan works because that is weak as hell.
for me its not the, not killing her that bothers me the most, its the author's choice on how he resolves the situation.
He set up a good moral dilemma for his mc and then instead of making his mc struggle and have to pick a route between what he considers both shitty choices, he makes the mc take a bullshit decision where he gets to have his cake and eat it too.
its even worse considering the other girl he spared was way worse, this was a more interesting choice and dilemma, and to give the mc a cop out, is such poor taste and writing. why even set up the moral dilemma in the first place then