I am on episode 3 and feel that Yoon is dumb or better selfish, how she can let the lady out when she knows how…
On the contrary, I believe she made the decision based on her instincts. Throughout the entire series, Sae Beom's character is one who is 'overly calm', a fast thinker, and who always puts herself first in the face of danger. I think she more or less managed to guess in the bathroom that the infected lady was only targeting her husband and wanted to test her theory (that she was obviously confident in), and she guessed right which greatly helped in ensuring that no one is killed off just because they are zombies. Imagine if she didn't test that theory, Si Hyun would probably be shot dead already.