Here Edmund uses the word “legitimate” to describe Edgar. On one hand, he could be referencing the fact that Edmund is the legitimate son of their father whereas he is the illegitimate bastard. However, according to the OED, “legitimate” can also defined as “conforming to the laws or to the rules”. Edmund could be saying that Edgar is “legitimate” because he did not have to break the rules in order to win his father’s favor, whereas Edgar plans on breaking many laws and rules in order to achieve power.

“legitimate, adj., adv., and n.” OED Online. Oxford University Press, March 2017. Web. 17 April 2017.