Zoë spares herself a fabulous weary smile, fights off the two dudes with a quickness, stands there looking awesome... And then gets shot through the chest. She remembers when Zoë's house caught fire, and how Daniel couldn't save her. Tamara walks onto the floor, out of the shadows, and notes how much it hurts when they shoot you. "By the way," she says, cocking that shotgun and firing a few more times: "You found me."
Why so serious, Adams? Continuity. The human need for continuity is the strongest thing in the universe, because we are lonely minds and can only live based on the facts that we know. When somebody cheats on you it only hurts a little because of possessiveness: It hurts mostly because the world in your head and the world outside are vastly different. You thought you were on solid ground and you weren't. It's ontological. It's shameful and embarrassing because the world is calling you a fool. What broke Amanda Greystone wasn't a mental illness relapse, it was finding out her entire family was filled with double agents who were completely unlike the people she thought they were.
In Tammy's version, she's a ghost: She died, she was resurrected, she lives here now. (Daniel's and Clarice's versions of Apotheosis agree on this, but they're all three wrong.) She has no other logical option than to assume that Zoë is the same thing, which is more than enough reason for vengeance. She has no context for Zoë's continuity -- born, died a few months later, got here by another route -- because the concept of Our Zoë is something nobody on the show can really get their heads around, because she is a miracle. The only evidence for, the anomalous Deathwalker thing, just makes Tammy's continuity more complete. Our continuity wants nothing more than to connect itself to itself, because that's the meaning of the word but also the only way we can live.
And knowing this fact about humans means a lot of power, for good or ill. For example, right now kidnapped Lacy is having her sense of continuity ripped apart, quite purposefully, by sexy Nestor: First the screaming abuses -- "You're useless! Worthless! "I don't even know why Clarice wants you here!" -- and then the sexy embraces, the protective muscles, curling around her up in the Willow attic: "Shh, it's okay. I won't let the others hurt you." Back and forth, good cop/bad cop, drugs in the water, and Lacy getting the floor ripped out from under her every time she finds a place to stand. Devastating.