Citazione Commento su I Cronache 21:16

But he was not warmed. Our Rabbis said, “He who disgraces clothing will ultimately be deprived of their pleasures.”¹Maseches Berachos 62b. [Dovid’s clothing did not warm him] because he tore off the corner of Shaul’s robe.²See I Shmuel 24:5. The Midrash [Aggadah states], Rabbi Shmuel son of Nachmeni said, that as Dovid saw the angel standing in Yerusholayim with his sword drawn in his hand, his blood became cold from fear.³That terror of that experience remained with Dovid, and from then on he could never again find warmth.

I believe that we must understand the verse thus: First of all, G'd announces that He would harden Pharaoh's heart as a result of which He would bring many plagues upon him. The plagues would prove counterproductive (initally) and make Pharaoh even more obstinate, adding still more sins to his already considerable culpability. Eventually, he would not even want to listen to what Moses and Aaron had been instructed to tell him. This is in fact what happened in 10,28 when Pharaoh bans Moses and Aaron from appearing again in His presence. G'd announces to Moses already at this point that when that stage will be reached it would be the limit of obstinacy that Pharaoh is allowed. Immediately after that ban G'd brought the final plague of the dying of the Egyptian firstborns upon him, as a result of which he would not only release the Israelites but expel them.

We find confirmation of this meaning of the word "My hand," when we read in Chronicles I 21,16 that David observed the angel of death with drawn sword in "his hand." In our instance, G'd refers to the fact that He personally would bring this plague upon Pharaoh by describing His activity as ידי, "My hand." This is why the Mechilta on Exodus 11,4 states that He Himself would be active as distinct from any of His agents. G'd also alluded to the destruction He would bring upon the various Egyptian deities at that time.