Why Did Kerrigan Kill Fenix in StarCraft: Brood War?

Kerrigan killed Fenix because, once their alliance had driven the UED off Korhal, she no longer needed him. She had used Fenix, Raynor and Mengsk to win back control of the zerg, and she wanted no one left strong enough to come after her. So in the Brood War mission True Colors she turned the Swarm on her allies, and Fenix and General Duke died. (StarCraft: Brood War)

Who Fenix was#

Fenix was a protoss Praetor, a veteran templar and one of the best warriors of his people. In the original StarCraft he fell to the zerg defending Aiur, and his broken body was put inside a dragoon, an armoured walker that let crippled warriors keep fighting. He fought beside Tassadar and Jim Raynor against the Overmind, and in Brood War he stayed behind with Raynor on Aiur to hold the warp gate so the refugees could escape to Shakuras. (StarCraft; StarCraft: Brood War)

The alliance he never trusted#

Late in Brood War, the UED, the fleet sent from Earth, has taken control of the second Overmind and most of the zerg. Kerrigan contacts Raynor and Fenix and asks for help. Fenix accuses her straight away of wanting to defeat the UED so she alone can rule the zerg. She says no: if the UED wins, they'll enslave the whole sector. (StarCraft: Brood War)

Fenix goes along because Raynor does, and so does Mengsk, who has lost Korhal to the UED. Together they destroy the psi disrupter that has cut Kerrigan off from her broods, raid Moria for resources and retake Korhal. Along the way Fenix says how strange it is to fight alongside his greatest enemy. (StarCraft: Brood War)

True Colors#

With the UED broken on Korhal, Kerrigan decides her allies have served their purpose. If they stay alive, they could become her enemies again. Her consort Samir Duran suggests a surprise attack while they're resting after the battle, and she orders it. The player's cerebrate gets six minutes before the allies notice. (StarCraft: Brood War)

Fenix isn't really surprised. He calls it a foul betrayal and says they were fools to go along with it. Kerrigan tells him he's right: she used him, he played along just as she knew he would, and the protoss are so headstrong and predictable they're their own worst enemies. She asks if he's ready to die a second time. Fenix answers that the Khala awaits him and that she won't find him easy prey. He leads his forces into the fight and is killed, along with General Edmund Duke. (StarCraft: Brood War)

Why she did it#

The mission spells out her reasons:

• She didn't need them any more. The psi disrupter was gone, her broods were back and the UED's grip on the sector was broken. (StarCraft: Brood War)
• They were a threat. Fenix and Duke led the strongest protoss and Dominion forces on Korhal. Killing them left both sides too weak to come after her. (StarCraft: Brood War)
• It was revenge on Mengsk. She tells him she lied about putting revenge second, reminds him he practically fed her to the zerg on Tarsonis, and leaves him alive to watch her rise. (StarCraft: Brood War)

Fenix wasn't her personal target the way Mengsk was. He died because he commanded an army that could stand in her way, which is what makes it so cold. A fair reading, though it is a reading, is that True Colors is where Kerrigan proves to herself she owes nothing to anyone from her old life. (StarCraft: Brood War)

How Raynor took it#

Raynor sees Fenix fall. Kerrigan mocks him, saying Fenix died the way every protoss hopes to, in combat. Raynor answers that Fenix died because she betrayed him, and swears that he'll be the man who kills her some day, for Fenix and everyone else caught up in her quest for power. In the final battle of Brood War, Artanis attacks her partly to avenge Fenix. (StarCraft: Brood War)

It never goes away. When Kerrigan rescues Raynor in Heart of the Swarm, having become zerg again by choice, he throws Fenix back at her, along with the millions she killed, and she has no answer. (StarCraft II: Heart of the Swarm)

Fenix after death#

In Legacy of the Void, the protoss find a Purifier, a machine warrior, carrying a copy of Fenix's mind made before the Great War without his knowledge. The copy wakes up believing it's Fenix, struggles with the truth and takes the name Talandar. He goes on to lead the Purifiers in the war against Amon. Fenix himself stays dead. (StarCraft II: Legacy of the Void)

Read more in the full Fenix biography, Brood War and why Kerrigan became zerg. (StarCraft: Brood War)