Edmund gives an angry Cornwall (“I will have my revenge”) the letter implicating his father in France’s intended invasion of England, arguing that betraying his father is for the greater good. Cornwall rewards him with his father’s title, figuratively adopting him while figuratively killing his father.
ACT III. SCENE V. Gloucester’s house.
Enter Cornwall and Edmund
CORNWALL I will have my revenge ere I depart his house.
How, my lord, I may be censured that nature thus gives way
to loyalty, something fears me to think of.
I now perceive it was not altogether your brother’s evil
disposition made him seek his death, but a provoking merit 
set a-work by a reprovable badness in himself.
How malicious is my fortune that I must repent to be just.
This is the letter which he spoke of, which approves him an
intelligent party to the advantages of France. O heavens,
That this treason were not, or not I the detector. 
CORNWALL Go with me to the Duchess.
If the matter of this paper be certain, you have
mighty business in hand.
True or false, it hath made thee Earl of Gloucester. Seek out
where thy father is, that he may be ready for our apprehension. 
[Aside] If I find him comforting the King, it will stuff his suspicion
more fully. [to Cornwall] I will persever in my course of loyalty,
though the conflict be sore between that and my blood.
I will lay trust upon thee, and thou shalt find a
dearer father in my love.