In your example I wouldn't go so far as to say you would be evil, perhaps just morally bankrupt or at the very least lacking in empathy.

You have someone like Jonas Salk who basically fit your example perfectly, except he decided to share his invention with the world instead of hiding it away as per your example.

You're free to not share your cure with the world, no one could force you, but I would question the motivations of someone who chose not to.