In the same interview with Vulture, Gunn said that the introduction of the character was the hardest part of writing the film. Unlike something like “The Avengers,” he doesn’t have the previous movies having their own Guardians to lean on. So he has to set up the best possible characters.
Thanos’ intro is particularly complicated because he shares the screen with Lee Pace’s Ronan. Gunn had to establish Thanos as a force to be reckoned with while not undermining Ronan, the film’s true villain. So Gunn decided to sacrifice The Other, Thanos’ minion who had previously mocked Loki during “The Avengers”. When the Other tried that on Ronan, he broke his neck. Gunn explains:
“I thought it was interesting, because we had the Other, who was obviously very strong compared to Loki, and then we saw Ronan wiping one of his** with him.”
Gunn recalls picturing the Other’s death more humorously than it did in the end; In the original script, Ronan tries to make excuses for the murder lightly, like a teenager who just scratched his father’s car. This scene is absent from the final clip and the scene is played more formally. However, the scene includes a moment of great humor, which Gunn calls “his favorite Nebula moment in the movie.” After Thanos threatened to “bathe the stars in [Ronan’s] blood “if Accuser failed him, a startled Nebula stood up, muttering, ‘Thanks, father. It sounds fair. “
https://www.slashfilm.com/994675/writing-thanos-into-guardians-of-the-galaxy-was-a-thorn-in-james-gunns-side/ Writing that Thanos becomes the guardian of the galaxy is a thorn in the side of James Gunn