Thanos Only Attacked Gamora's Planet to Defeat His Secret Nemesis
While Thanos' attack on Gamora's home planet of Zen-Whoberi was the darkest day of her life, the true horror of the situation was that it was just one of many attempts by the Mad Titan to weaken and defeat his secret nemesis, the being known as Magus. While Thanos' early years left him as a pirate king in charge of an army of brigands, his commitment to the cosmic embodiment of Death saw him quickly take on larger significance in the galaxy, earning him a class of enemy he wasn't yet able to defeat outright.
Thanos' attack on Zen-Whoberi wasn't based on blood lust, his love of Death, or even his MCU counterpart's desire for universal balance, but rather the desire to prevent Magus growing stronger. After massacring the pacifist population of the planet, Thanos "adopted" Gamora as her species' only survivor, raising her as the deadliest woman in the galaxy and unwittingly creating a hero who would stand against his schemes again and again. But while the significance of this event would shape her entire life, Gamora's entire planet was wiped out as just one move in a cosmic chess match.
The Magus is a dark version of the heroic Adam Warlock who draws his staggering power from an organization known as the Universal Church of Truth - a galaxy-spanning religion which converts its adherents' belief into pure power. That power is transmitted to its leaders, allowing them to conquer more worlds and win even more converts. In 2019's Thanos #1-6, Ariel Olivetti and Tini Howard chart Thanos' discovery that Magus will one day be his greatest threat. Knowing his followers are not yet capable of defeating Magus' church, Thanos characteristically resolves to instead slaughter the planets in Magus' path, slowing the growth of his organization so that Thanos can outpace the Universal Church of Truth and crush it beneath his heel.
While Thanos hid the truth about Magus from his crew, who would have pounced on any sign of weakness, this battle of wills continued for years, and Thanos' training of Gamora was actually intended to prepare her to one day slay Magus on his behalf. Of course, Gamora ultimately becomes a hero, and in an ironic twist of fate, instead slays Thanos and ends up rescuing a de-aged, mind-wiped version of Magus, with the events of the comic revealed as her relaying her own story to the young being, hoping to inspire him to take a different path than the evil which could await in his future.
Thanos is a being of terrible metaphysical significance, and for many years Magus was his opposite number, pushing him to new atrocities and accomplishments in their battle for supremacy. The attack on Zen-Whoberi combined the destinies of Gamora, Thanos, and Magus, leading to an outcome none of them could have foreseen. Cruel as Thanos is, his attack on Gamora's home planet wasn't random or done on a whim, but was rather a calculated move in defeating the perhaps greater evil of Magus - a being who Gamora would later choose to try and redeem in a definitive rejection of her monstrous father.