I find the Midrash about Pharoah's heart fascinating. One of the first brilliant insights people who are introduced to the Torah come up with, is: G!d hardens Pharaoh's heart. How can he then punish Pharaoh and his people. This is almost always stated as a new and brilliant original discovery. But as with most of the problems in the Torah, the Rabbis were there thousands of years ago and raised all of the same questions. We may not agree with the answers but we have to understand that the Jews have always read Torah with a critical if believing mind.
G!d in his great power is able to harden, even a "non-existent" heart. Some hearts (read mind not emotion) are so dense that they can never be softened. Pharaoh was so used to having everyone agree with him that even when G!d warns him and fufills the warnings Pharaoh hears G!d as encouraging Pharaoh's stubborness. So G!d's warings are given as Chesed (lovingkindness) but are read by Pharaoh as Gevurah (strict justice). Pharaoh still believes he will define and enforce justice no matter what G!d says. So just bywarning (thru Moses), G!d hardens Pharaoh's heart.
I just made it. I may rewrite on Thursday.