Why did Pharaoh suddenly acknowledge Hashem's righteousness, and his own wickedness?
Hadar Zekenim: Because Hashem had performed with him a great kindness, by warning him in advance. 1
Moshav Zekenim #1: Because when he saw how the G-d-fearing Egyptians took their animals into the house, and were spared, he realized that, had he and his people feared G-d, they too would have been spared.
Moshav Zekenim #2, Da'as Zekenim, Hadar Zekenim and Riva: Because Hashem had mercy on the flocks and instructed them to save them, but he and his people did not heed Him, and left them in the field. 2
Moshav Zekenim #3 (to 8:19): He said 'I sinned,' because the hail ruined the crops, the trees, the animals and the people - like no other plague.
Refer to 9:19:1:1*. Maharal (Gevuros Hashem Ch. 33, p. 125) - Pharaoh realized Hashem's righteousness, in that He told them in advance to bring their animals into the houses. Refer to 9:19:151:1.
Hadar Zekenim: Also after Arbeh, Pharaoh would say, "Chatasi La'Shem Elokeichem v'Lachem" (10:16) - 'I performed evil with you, but you performed Chesed with me!' It seems that Pharaoh's ego was being systematically deflated by the force of the Makos.
