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Natasha was the only one who came for vengeance. Tony was ready to, Clint was close to it, and Thor might have been if he weren't more interested in rescuing his little brother that was and had disappeared behind this heartless trickster.
Loki had taken Clint and taken the free will that had always been his trademark, the one thing that hard living and almost dying and killing too many souls had never taken from him, the one thing he'd offered to Natasha that had the power to bring her in to SHIELD in the first place.
It was unforgivable and unacceptable. Regimes fell every day and Natasha wasn't actually all that interested in avenging the earth if it fell, just one man: Clint.
So she put off her stealth and her espionage mindset and kept the deadliness of her namesake. She became a soldier. She waded into war. She took her vengeance and became an Avenger.