Yeah but making him a human, especially one that coonfirms everything Erik hates about human's would have made Erik's ark much stronger. Making him a mutant (even one he didn't know about to begin with) dilutes that, because in the other films Magneto has such a strong hatred for preducial humans. Also making this mutant uber-powerful seems like a move by the producers to go for the lowest common demnominator in blockbuster movie making. It ght have been interesting to see this Nazu perform these experiments on mutants, but then gradually change as he learns more about thm. He wouldn't totally sympathize with them, but just as he seems to be getting close to doing so, Erik kills him.
One of my favorite scenes in the first film is when Erik is looking at the Statue of Liberty and telling Rogue what it meant to him. The Erik in this movie seems so different from that awesome character we had.