This might be where Thanos got his rage from.
70-year-old comic book legend Jim Starlin has no love for Marvel. In an interview with Vulture, Starlin reveals that he has had ups-and-downs with the comic book publisher for decades, and his hatred for them has only increased in recent years.
“I’m not an angry person, which you can probably hear from just me talking to you,” Starlin begins. “But Marvel tends to bring out the worst in me, at times.” Starlin has quit his job at Marvel six times, but has always returned because of his love for his characters, namely Thanos, a villain that he created along with the Infinity Gauntlet. Both Thanos and the powerful glove are featured heavily in the new Avengers: Infinity War film, which is mostly a Thanos based film.
Starlin's beef is mostly centered around the comic book division of Marvel, and he has little to no issue with Marvel Studios, which paid him what he deemed as an insufficient amount of money to use his characters. Although he may have been unsatisfied with what the movie studios paid him, his real qualm is with the comic book sector, which he feels has comprised the integrity of one of his stories. According to Starlin, while he was working on an exclusive Thanos based storyline for a graphic novel story arc, Marvel allowed another writer to create a story that was very similar to his own. To make matters worse, Marvel planned to release the other story before his, making Starlin look unoriginal. While the Thanos creator claims that he doesn't necessarily believe his idea was stolen, Marvel was given enough time to at least correct the situation and make the stories differ, which they did not do. He claims he will never work for them again.
At the end of the day, Starlin is still excited to see Thanos appear in the new Avengers film, although his relationship with Marvel is over.