To give the full context:

Subaru Natsuki is the black-haired guy, and he has the ability to rewind time. Because of this, he has privileged knowledge that the equivalent of a satanic cult is about to invade the territory where his friends live and slaughter everyone there.

The green-haired woman is Crusch. Besides being an extremely powerful knight and one of the candidates for the kingdom's throne, she possesses a Divine Protection that detects lies, meaning she knows by default that Subaru is telling the truth—or at the very least, genuinely believes everything he's saying.

Subaru is asking Crusch to help save his friends, including Emilia, who is Crusch's political rival for the throne and the girl Subaru loves. At that point, Subaru knew for certain that if he didn't make it back in time with reinforcements, everyone there would die. He was desperately trying to obtain Crusch's help, especially since he had already spent several days at her mansion while recovering from his damaged mana gate and had even shared meals with her.

Even so, Crusch firmly refuses to send troops because:

1. Crusch and Subaru belong to rival royal camps competing for the throne. If the leader of Subaru's camp can't protect her own people without having one of her retainers beg a political rival for help, then she isn't fit to rule.
2. Crusch herself was preparing to lead an army against one of the Three Great Mabeasts in the near future. She wanted to dedicate 100% of her forces to that battle because, from her perspective, it was likely to be the most important battle of her life, and splitting her resources simply wasn't worth the risk.

She even says that if innocent people were to die, it would be the result of Emilia's own incompetence.

Subaru was absolutely furious and thought her reasoning was outrageous. But looking at the situation as neutrally as possible, was Crusch actually in the wrong?

(She does help Subaru in later loops, but in this first one specifically, she refuses.)

Morality of refusing to help a rival political candidate avoid being literally massacred?