"Your planet was on the brink of collapse. I’m the one who stopped that. You know what’s happened since then? The children born have known nothing but full bellies and clear skies. It’s a paradise." "Because you murdered half the planet!"
Zen-Whoberi was the homeworld of the Zehoberei, a race of green-skinned beings. Like many planets, it suffered from overpopulation which brought the planet on the brink of collapse; poverty was commonplace, many children were homeless, and people constantly hounded the planet for scraps.
This changed when the planet was invaded by Thanos with his Chitauri army and the Black Order. Like they usually did, the invaders proceeded to wipe out half of the planet's population. According to Thanos, despite this horrific act of mass murder, the planet was effectively saved; the remaining Zehoberei were able to thrive more easily on the planet without succumbing to starvation or poverty, and the world's resources no longer dwindled.
One young girl of Zen-Whoberei, Gamora, lost her parents during this massacre. She was taken from the planet by Thanos as his adopted daughter to be his assassin. Upon discovering what Thanos had done to her home planet, Gamora began to hate her adoptive father.
While Thanos stated that that Zen-Whoberi became a "paradise" after culling half of the population, the RussoBrothers implied that the former's recollection isn't entirely accurate after committing the slaughter.