Thanos was originally going to sacrifice Nebula instead of Gamora

In the MCU film Avengers: Infinity War the scene on Vormir could have potentially played out differently. Thanos has issues this is obvious. Though he seems delusional on alot of levels that still doesn't dismiss that he is indeed capable of love. Like most individuals with psychological issues his concept, understanding, and most of all his execution of love can be warped and misunderstood by others. As we all observed he actually did love Gamora, and in addition to that Nebula. The fact is Thanos loved both of his daughters this training and conditioning them to defeat anyone or anything. He did this out of love and fear of losing his daughters to a cruel universe. When The Red Skull informs him of the sacrifice the reason he's quiet for so long is because he was deciding. Previously on Knowhere he saw that Gamora grieved him when she thought she had killed him. He knew that's something Nebula would never do. In fact he himself knew that between his daughters Nebula would be the 9ne to kill him. He knew this because he trained her and built her to be that way. Also he knows Nebula wants to die. And as her father he wants to do something right by her by giving her peace her existence can't bring. While on the other hand he wants Gamora to have a simple life with Quill. He wants her to have what he always wanted but could never have. It was the fact that Gamora broke his heart by saying he didn't love her. He was genuinely hurt and confused by her words. As a father he doesn't perceive his actions in the same manner she does. Which is why there is so much regret about hu is action later on in the film. Nebula was the most like him. So killing her would be in a meta way killing the old version of him once the mission was completed. Nebula is everything he is and has had to be while Gamora is everything he is fighting for. In conclusion on Vormir Thanos was initially gonna go back to his ship detain Gamora and take Nebula down to sacrifice for the soul stone.