Power means extream authority to judge. A person may judge by his own will or with the help of councellors to reach the truth. In the case of King Lear, being the king, Lear neither have any one. Due to the exccess of power he was unable to see the truth behind the curtain. He was unable to see that was is there in the mind of his own two daughter. He neglacted or avoid the human nature of flattery in the pomp of power, he worked as a blind and distribute his kingdom among two daughters who speeks only what seems him pleasent. His power made him blind that he didnt find the truth and love there in the heart of his younger daughter.
So according to my opinion, King Lear become blind to recognise the human behaviour due to the power, and his such kind of demarit become the cause of his downfall. This Hamartia become the reason for King Lear to be a great tragedy.