Sacrifice of Gamora

"Tell me what it needs." "To ensure that whoever possesses it, understands its power. The stone demands a sacrifice." "Of what?" "In order to take the stone, you must lose that which you love. A soul for a soul."

- Thanos and Red Skull

The Assassination of Gamora was the result in order for Thanos to obtain the Soul Stone.

Background
To be added

Assassination
"All my life I dreamed of a day, a moment, when you got what you deserved. And I was always so disappointed. But now, you kill and torture and you call it mercy. The universe has judged you. You asked it for a prize and it told you no. You failed. And do you wanna know why? Because you love nothing. No one. Really, tears?" "They are not for him." "No. This isn't love." "I ignored my destiny once, I can not do that again. Even for you. I'm sorry Gamora." "No!"

- Gamora, Red Skull and Thanos

After arriving on Vormir, the Stonekeeper lead Thanos and Gamora to the location of the Soul Stone. The Storekeeper informed them that the in order to obtain the stone, the possessor must make a sacrifice of the thing they love. To Gamora's amusement, she laughed at her adopted father, claiming that he loves nothing and this is the universes way of punishing him. However, Thanos was in fact deeply saddened as Gamora was the only thing he loved. Gamora then realized that Thanos loved her, to which she rejected. Thanos then grabbed her and dragged her towards the edge of the cliff, despite her screams, and threw her off, ultimately killing her. Once she had died from impact, Thanos was granted with the Soul Stone and headed to Titan meet up with his Black Order.

Aftermath
"He mourns." "What does this monster have to mourn?" "Gamora. He took her to Vormir. He came back with the Soul Stone, and she didn't." "Okay, Quill, you gotta cool it right now, you understand? Don't, don't, don't engage, we almost got this off!" "Tell me she's lying. Asshole! Tell me you didn't do it." "I had to."

- Mantis, Drax the Destroyer, Nebula, Iron Man, Star-Lord and Thanos

Thanos, now equiped with four Infinity Stones, arrived on Titan where he was ambushed by the Avengers, the Guardians of the Galaxy and Doctor Strange. After an immense battle, the heroes were able to subdue Thanos and were about to remove the Infinity Gauntlet until Star-Lord began to ask about Gamora. Nebula then revealed that Thanos must of killed her, as he possesses the Soul Stone. Star-Lord then became furious and lashed out at Thanos, who felt bad about killing his daughter, causing Thanos to break free and defeat the heroes. When he finally collected all the stones, he snapped his fingers and ripped out half of the universes population. Thanos found himself in limbo, where he met a young Gamora and told her he finally completed his mission. When she asked what it cost, he solemnly replied, "Everything."