There was a script where he was the primary villain of the movie. (superfans.website)
However, it is nice to finally be able to fully explore the chosen villain of a movie, without being worried that its menace will be diluted. (oracleoffilm.com)
I should probably address the decision to make Red Skull and HYDRA the main villains of the movie as opposed to the Nazis, a decision that has attracted a whole lot of, uh, spirited discussion. (io9.gizmodo.com)