For those unfamiliar with the background of this question, a brief synopsis of the story found in the book of Exodus beginning in chapter 3.
The Israelites were enslaved in Egypt. Moses after a not too well thought out action found himself a fugitive in a far land, where he met the LORD, who called him and assigned him to return to Egypt and free His people. With some reluctance Moses packed up his family and started back, but before he'd gone too far the Lord gave him a final warning:
"When you get back to Egypt, see that you perform before Pharaoh all the wonders that I've put in your power, but I will harden his heart so that he won't let the people go!" (Exodus 4:1) Not too encouraging!
Thus begins a long dramatic story of the contest between the LORD and the gods of Egypt with Moses performing all sorts of signs and wonders and Pharaoh seeming to relent followed by the statement that "Pharaoh's heart hardened" or something similar.
Before we get too far, we should notice there are 21 references to the hardening of Pharaoh's heart in chapters 4-14 of Exodus. There are 4 different Hebrew words used and there is a great variety of expressions:
· In 10 references, the LORD (or the pronoun "I") is the subject -- 4:21; 7:3; 9:12; 10:1, 20, 27; 11:10; 14:4, 8, 17:
"I will harden Pharaoh's heart"; "The LORD hardened Pharaoh's heart."
· In 3 references, Pharaoh is the subject -- 8:11, 28; 9:3:
"Pharaoh hardened his heart."
· In 7 references, Pharaoh's heart is the subject -- 7:13, 14, 22; 8:19; 9:7, 35; 14:5:
"Pharaoh's heart hardened" or "Pharaoh's heart was hard."
As about half of the references refer to the LORD doing the hardening and about half seeming to give Pharaoh the blame for hardening his own heart, we have to ask a second question -- who is responsible, the LORD or Pharaoh?
Though we're told twice that the LORD promised to harden (4:21 and 7:3) before we're told that Pharaoh had a responsibility (7:13, 14), we're not to suppose that Pharaoh is simply behaving like a robot. We see a hard-heartened self-willed Pharaoh already in chapter 5 when Pharaoh challenges Moses.
"Who is the LORD that I should obey His voice to let Israel go? I do not know the LORD and besides I will not let Israel go!" (5:2)
The LORD takes the challenge as we read in the following chapters. The words "I am the LORD" or something similar are repeated over and over, apparently in reply to Pharaoh's demand, as the LORD strikes Egypt over and over with plagues. We could say that the whole rest of the story tells of the consequences of Pharaoh's rejection of the LORD.
It's in 9:16 though where the LORD through Moses tells Pharaoh, "For this cause I allowed you to stand (or made you stand) in order to show you my power and in order to proclaim My Name through all the earth!"
So this is the answer to our "why" question. God hardened Pharaoh's heart in order that His name would be proclaimed. If I may put this in other words, God used the hardness of Pharaoh in order to let the world know who He, the LORD is.
To answer the question as to who is responsible, we need to go outside the story. The Apostle Paul comments on this passage in Romans 9:14-18. There he makes clear that God is sovereign in all His actions whether He has mercy or hardens.
But as I mentioned above, Pharaoh appears in the story as a hard-hearted, self-willed sinner. And throughout the story he seems to be held responsible for his words and actions.
There appears to be a theme throughout the Scripture: God hardens people, but the people He hardens have already chosen to be hard. We don't read of God hardening people who have good intentions or motives. Perhaps Paul's words in Romans 1:18ff best describe the "process." People know something of God and His will but reject it. It is then that God hardens, or as Paul says, He "hands them over" to the consequences of that rejection- further hardening - a mind incapable of making correct moral decisions.
God is the Sovereign Ruler of the universe. According to Romans 8:28, "He works all things together for good to those who love God, to those who are called according to His purpose." The "all things" includes even the actions of those who are opposed to Him.