כִּ֤י עַתָּה֙ שָׁלַ֣חְתִּי אֶת־יָדִ֔י וָאַ֥ךְ אוֹתְךָ֛ וְאֶֽת־עַמְּךָ֖ בַּדָּ֑בֶר וַתִּכָּחֵ֖ד מִן־הָאָֽרֶץ׃
Pois agora, se Eu estendesse a Minha mão e te ferisse a ti e ao teu povo com pestilência, serias exterminado da terra.
Rashi
destroyed Heb. נכחדו, disappeared and perished, like (Exod. 9:15), “and you would vanish (ותכחד) from the earth.”
Chizkuni
ואני אחזק את לבו, “I will harden his heart;” this does not mean that G-d deprives Pharaoh of the ability to become a penitent and to reverse his attitude and cooperate with G-d’s commands if he so wills it. The line must be understood as follows: “I will give him additional courage so that seeing My miracles he will not die from fright until all My miracles will have been performed.” This is also how we must understand Exodus 73: ואני אקשה את לב פרעה, “I will toughen Pharaoh’s heart;” he will not collapse from fear.” If you needed proof for this interpretation consider Exodus 9,15: כי עתה שלחתי את ידי ואך אותך, “for now I will stretch out My hand and smite you with pestilence.”