Responses
The main reason why is that yui herself did not kill anyone. Anytime a death happens it's when the virus zombie takes over. And zombie as well doesn't act with malicious intentions. She sees infecting others as becoming friends. So she technically can't be a bad person if she doesn't understand why it's bad. It's still dumb but that's the reasoning

Why would the main character hesitate??? I get the girl wants to make friends but she killed THREE THOUSAND PEOPLE ALONE HERE!!!! WHAT DO YOU MEAN SHE'S NOT A BAD PERSON??? THATS EXCUSABLE???? The village people i could excuse she acted in self defense and they were buggin but the people on the island were chilling??????? And they actively liked her because she saved a child! This raised my blood pressure so much I hope she dies soon and painfully, main character and his crazy sister too!