The planet of
Zen-Whoberi is 'balanced' by
Thanos, with half the native
Zehoberei being slaughtered by his forces.
This rapid depopulation would eventually lead to the complete exstinction of the species.
During the battle, Thanos was impressed by the courage of a Zehoberei child,
Gamora , who reminded him of his daughter
Black Swan, who had recently died. He therefor chose to kidnap the child, and raise her as his daughter.