Edmund
Quote 7
EDMUND
[…] Edmund the base
Shall top th' legitimate. I grow, I prosper. (1.2.21-22)
Because Edmund feels he's been shafted by society and his father (for being an illegitimate and second-born son), he justifies his disloyalty and scheming against his family. Edmund feels entitled to "grow" and "prosper" at the expense of his father and half-brother. For him, there is no such thing as family loyalty or duty.