Duke of Cornwall
Cornwall, Regan's husband, represents abuse of power at its worst. Lear is certainly one for getting angry and making poor decisions, but Cornwall's anger has a sadistic edge. He enjoys causing other people pain, and he likes being in power because then nobody is allowed to stop him.
You can tell Cornwall is mean and power-hungry when he puts Kent in the stocks for failing to show a little respect, but the real stuff comes out when plucks out Gloucester's eyes for giving King Lear shelter near his castle. Cornwall cold-bloodedly explains that he wants to hurt Gloucester just for the pleasure of it, and that nobody can stop him:
Though well we may not pass upon his life / Without the form of justice, yet our power / Shall do a court'sy to our wrath, which men / May blame but not control. (3.7.27-30)
In other words, Cornwall knows he can't get away with literal murder, but he can still get away with figurative murder. So this is the essence of Cornwall: power is just a tool to gratify sadistic tendencies. Ick.Duke of Cornwall Timeline