What is more terrifying?
Is it facing a powerful enemy, one you can meet head-on, locking eyes with them in a display of bravery and defiance even in the face of certain death? Or is it confronting an adversary who can silently take lives all around you, their presence imperceptible?
Seeing each member of the group fall, one by one, undoubtedly shattered the leader's morale. His ability to think rationally, with cold logic, had been shaken. The only thought that occupied his mind was survival, whether it meant using the wounded comrade in his hands or even sacrificing Keila for it.
That decision acted as a trigger in the leader's mind, as if a mental lock had been undone right at that moment. And so he did it: abandoning the injured team member, he simply said, "Keila, the mission has failed. Let's retreat now."