The Marvel Cinematic Universe has only ever used its comic predecessors as a starting point to tell new and different stories. The newest trailer for Avengers: Infinity War highlighted one specific change that the new movie will make to the Infinity Gauntlet comic that inspired it. It clarified, to a point, exactly what Thanos' motivations are for hunting down the Infinity Stones and what he plans to do as soon as he has them all. He's going to wipe out half the universe because he doesn't feel the universe is currently in balance.
The trailer doesn't go into exactly why things are not currently "perfectly balanced" but it appears that we'll get our answers in Avengers: Infinity War as the trailer shows us a brief moment that looks to be part of a flashback sequence, Thanos walking with a child who is very likely Gamora. This may be part of the story that Gamora is telling at the beginning of the trailer. It's a quite specific goal for Thanos to have to wipe out half of the universe. Clearly, something happened to him in the past that made him decide that this was a necessary thing to do.
It seemed fairly clear early on that Thanos' motivations would be different in the Marvel Cinematic Universe than they were in the comics that inspired it, we just didn't know exactly what they would be. In the comics, Thanos has a somewhat more mundane reason for doing what he does. He's trying to impress a girl. Specifically, he's trying to win the affection of the personification of Death by taking control of all life in the universe via the Infinity Stones. Interestingly, the trailer does make one passing reference to the comic inspiration that we see in the trailer. Gamora mentions that if Thanos gets all the Infinity Stones he can wipe out half the life in the universe with a snap of his fingers. We know this is true because, in the comics, he did exactly that.
In the comics eliminating half of all life was a means to an end, but here, it appears that doing so is the actual goal. Perhaps Thanos is just worried about universal overpopulation. Maybe galactic food supply is getting low. Whatever it is, he thinks the galaxy would just be better off with fewer people in it. He specifically states that half of humanity will survive, so he's apparently going to be fair about which half goes and which half stays. He's not going to destroy all humans just so every member of some other planet can live. Although, since they tried to stop him, I'm guessing half the Avengers won't get to hang around.
