I'm not sure that's true - we know Khan in TWOK was marooned on a planet by Kirk, and that he considers himself a superior being to Kirk - so we know he was humiliated by his imprisonment. We know that this humiliation was compounded once the planet became a wasteland where he and his people had to struggle to survive for decades because no one bothered to check on him. When he tells that story, you cannot help but imagine him, day after day, year after year, ruminating on his humiliation, the injustice of it all, especially to a being born to rule. We know his beloved wife was killed and he considers Kirk personally responsible. Also, having the same actor reprising the role means that if you know Space Seed you can believe that it is that character's history, so the possibility is there to bring that knowledge to the movie.
This Khan wasn't the Khan I knew. I don't know if he was involved in the Eugenics War, no Eugenics war was mentioned. I don't know why he was outlawed, put in cryogenic sleep and launched into space. I don't really know how he feels about regular people other than that he considers himself superior to them - but that seems a rather objective assesment on his part. That is, it doesn't have any emotional color. He never mentions despising regular humans, wanting to destroy or rule them, in fact he never mentions wanting anything except to be with his people - so he seems a ... smaller character. Cumberbatch's choice to play him so cold also means he's a bit of a cypher. I don't know how he feels about anything except that he loves his people and will do anything to save them, which is a fairly generic motivation. Montalban's Khan had very specific motivations, even if all you know is what you learn in the movie itself.