Massacre of the Zehoberei

"When Thanos took my home world, he killed my parents in front of me. He tortured me, turned me into a weapon."

- Gamora to Star-Lord

The Massacre of the Zehoberei was a mass murder carried out by Thanos in his quest to bring balance to the universe.

Background
"Titan was like most planets. Too many mouths, not enough food to go around."

- Thanos to Doctor Strange

Millennia ago, after exhausting their finite resources to deal with overpopulation, the eponymous inhabitants of the planet Titan struggled to come up with a solution. When the outcast Thanos proposed massacring half the population at random, thus allowing the other half to thrive, the people deemed his proposal far too extreme and dismissed him.

Eventually, the Titans, save for Thanos, were exterminated by the cataclysm they feared would happen and the planet was rendered unrecognizable. Convinced that his plan could have saved his people, the guilt-ridden Thanos set out on a violent campaign to prevent other planets from experiencing Titan's fate.

Thanos' ideology, along with his growing power and vast armada of both Chitauri and Outriders, cemented his reputation as a feared warlord, further convincing him that the only way to bring balance to the universe was to exterminate half of all life. He began seeking out planets suffering from overpopulation and overwhelming their defenses before massacring half the populace. Eventually, his quest led him to the planet Zen-Whoberi, which was on the brink of collapse.

Massacre
"Zehobereians, choose a side or die. One side is a revelation. The other is an honor known only to a few."

- Ebony Maw

Leading the Black Order and a contingent of Chitauri soldiers, Thanos successfully invaded Zen-Whoberi and rounded up the members of its populace. Ebony Maw then ordered that the population be divided evenly into two groups, one that would be spared and another that would be executed.

In the midst of the chaos, a young girl named Gamora tried to fight off the Chitauri soldier restraining her so she could find her mother. Thanos, who witnessed this act, took her aside and explained his beliefs through the use of a switchblade that he balanced on his finger. He then asked Gamora to do the same just as Maw ordered the Chitauri soldiers to open fire on the second group.

As the soldiers carried out their orders, Gamora started to turn around only for Thanos to bring her attention back to the switchblade and compliment her when she managed to balance it just as he did.

Aftermath
"We were happy on my home planet." "Going to bed hungry? Scrounging for scraps? Your planet was on the brink of collapse. I was 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."

- Gamora and Thanos

Extreme as Thanos' methods were, the surviving Zehoberei overcame the collective trauma and began thriving. The Mad Titan proceeded to use this as justification for his horrific methods.

Gamora, meanwhile, was raised to serve as one of Thanos' enforcers. Unlike her adoptive siblings, Thanos grew to genuinely care for her and she became his favored daughter while he built her up as "the deadliest woman in the galaxy".

When she discovered what Thanos had done to her home planet, Gamora began resenting her adoptive father's actions and eventually betrayed him, putting her training to good use as a Guardian of the Galaxy.

Appearances
In chronological order:
 * Avengers: Infinity War (flashback)
 * Guardians of the Galaxy (mentioned)
 * Avengers: Infinity War (mentioned)

Trivia

 * In the comics, the Zen-Whoberi were completely annihilated by the Universal Church of Truth when the race declined the offer of joining the cosmic cult.