The trigger is the quick series of deaths Phil metes out at the beginning of the episode on a mission. As he later tells Elizabeth, such killings on behalf of their work are easier for her than for him. She takes some exception to this, but we've seen this to be manifestly true.
The accelerant, which nearly pushes Phil over the edge, is daughter Paige's immersion in her new-found religion, and in particular her donation of $600 to her church. Elizabeth is not at all happy about this, but Phil, upset about his killings, first screams at Paige, and later goes to the church to confront the pastor. The ensuing scene ended the episode, as was so tightly drawn that I wasn't sure if he was going to kill the pastor right then and there. The man who doesn't like killing for his beloved Soviet Union was this close to killing the pastor for what he did to Phil's daughter, and maybe because of his infuriating believe in "his" God, too.
Phil manages to walk out of the church, but this interlude shows, like nothing else before on the show, how fine a line Phil walks between his family and his job. His fury at what Elizabeth has been doing has nothing to do with his job. It stems from Phil's feelings of protectiveness as a father. From his and Elizabeth's point of view, Paige's finding religion is a development that goes against every grain of their Soviet being. But what really makes it hurt is the love they feel for their daughter - a love that, on the one hand, is part of their pretense of being a normal American family, but, on the other hand, is as real as the love that any parent feels for his or her child.
Meanwhile, there are lots of good developments in the spy part of the story. Gaad threatening Oleg was unexpected and bracingly out of character, providing an analog to Phil being so out of character himself. It will be interesting to see where that goes. Martha, goaded by Phil's playing the doctored tape to her, has now been positioned to do more serious damage to Stan and Gaad. And Stan's wife telling him point blank that she was leaving for a weekend tryst was just what he deserved. Ok, that wasn't part of the spy story per se, but it's certainly part of the life of a spy.
The Americans continues to be original, exceptional television in its second season.
See also The Americans 2.1-2: The Paradox of the Spy's Children ... The Americans 2.3: Family vs. Mission ... The Americans 2.7: Embryonic Internet and Lie Detection
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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