In the mission, Zagara hunts the map for eggs. Kerrigan's forces can kill her. She respawns, but this gives time for Kerrigan to collect more eggs. Kerrigan immediately collects eggs upon touching them, while Zagara takes some time.
At the end, Kerrigan's collected eggs all hatch at once, delivering hundreds of zerglings and banelings. Upon defeat, Zagara pleaded for mercy and was forgiven. She joined Kerrigan's forces.