Upon showing this letter to the gullible Gloucester, he ordered to have a contract out for Edgar's head.
In [Act 5 scene 3] Goneril has poisoned her own sister, in hopes of capturing Edmund's love.
However, Edgar and Cordelia are exceptions to this generalization.
There seems to be a great disparity in the way the older generation and younger generation views loyalty. Being a man of great power and authority, he cannot fathom having someone lie to him.
It is this naïve approach that leaves him vulnerable for betrayal.