Consider, for example, what happens when the guy in the parts plant realizes that Elizabeth is not who she says she is, and Elizabeth sees this in the guy's face. The smart thing to do would be to kill this guy, after Elizabeth and Phillip get what they want from him. He shows Elizabeth pictures of his family, in a last ditch attempt to get Elizabeth not to kill him. And it works.
Instead of killing him, Elizabeth takes the photo of the guy's son, as a way of letting him know what would happen to the guy's family if he told someone about Elizabeth. Was this the smartest thing for Elizabeth to do? Probably not, because the guy could still at some point in the future reveal to someone what she did in the plant - and maybe he will, before the season or the series is over. But Elizabeth didn't spare him because that was a smarter plan than killing him. She saved him because she couldn't bring herself to murder a father in cold blood, for even the best of reasons.
After all, she's still torn up about the murder of Jared's parents and sister, and her actions tonight regarding him are also guided more by human feelings than by obligations or commitments she made. She promised Jared's mother, years ago, that if ever anything happened to her and her husband, she would give Jared a letter Jared's mother wrote, explaining to him who they were really were. But after seeing Jared tonight, in one of Elizabeth's many good disguises, she decides not to give him the letter, and burns it instead. She realizes that the letter would do Jared no good at all, and only further cut up his life. So she acts as a human being not as an agent.
It is this humanity which both Elizabeth and Phillip have, which comes out not only whenever their own children are involved but now also for other children - Jared, and the guy at the plant's son, tonight - that is both their greatest strength and their greatest weakness.
Nina, so far, has betrayed no such conflict between her obligations and whatever she may feel for Stan. It will interesting to see how far she can get with this masquerade and her great achievement of getting Stan to love her. Phillip has done an amazing job of it so far with Martha, but he's much more of a pro at this than Nina.
See also The Americans 2.1-2: The Paradox of the Spy's Children
And see also The Americans: True and Deep ... The Americans 1.4: Preventing World War III ... The Americans 1.11: Elizabeth's Evolution ... The Americans Season 1 Finale: Excellent with One Exception
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