Rashi on Exodus
והכבד את לבו HE ALLOWED HIS HEART TO BE HARDENED — The word הַכְבֵּד expresses doing something (the Hebrew infinitive but having a past meaning) just as, (Genesis 12:9) “going (הלוך) and journeying (ונסוע)”. So, too, (2 Kings 3:24) “and smiting (והכת) Moab”; (I Samuel 22:13) “and enquiring (ושאול) of God for him”; (1 Kings 20:37); “smiting (הכה) and wounding (ופצוע)”.
Rashbam on Exodus
HE MADE IT HARD [HACHBED]. Because this plague was great, his mind did not strengthen by itself, rather, he affirmatively acted wrongly to harden his mind.
Sforno on Exodus
כי היתה הרוחה, even though the evil had not been removed totally, seeing that the stench of the dead beasts remained and putrefied the atmosphere, and these beasts remained in the river as a possible threat in the future.