But WHY was he the alpha male, why was he intelligent.
Why do apes learn sign language?
If you think of it this way: The virus caused the infected to regress to a more animalistic, and cannibalistic, state. once they had eaten or infected the entire population of Manhatten, save the Doc, their food supply had disappeared. A wild animal faced with a large predator population or, in this case, starvation will begin to learn new ways of hunting or turning to new foods.
That's why Panda bears eat bamboo. They used to be omnivores but then they started watching reality TV and decided it was too much work chasing down something with legs and just sat on the couch all day.