Unlike Achashverosh of Persia in the Purim story, Pharaoh was not a weakling who had no mind of his own.
Once he made up his mind, he could not be budged. He could have let the Israelites go much earlier but he hardened his heart to such an extent that nothing could penetrate his obstinacy.
When the text says that God hardened Pharaoh’s heart (Ex. 10:20 etc.), this indicates, according to Ibn Ezra and other commentators, “When one wishes to do wrong, opportunities are given to him”. But if someone has an inclination to be good and to act morally, God also helps them along.
Pharaoh could make up his mind so strongly and be so obdurate that he might have applied to himself the words that some people apparently display on their office wall or desk, “Don’t confuse me with logic: my mind’s made up!” In his case, the suffering of others was unable to penetrate his heart.
“Don’t walk to me about compassion,” he would have said, “Can’t you see I’m hardhearted?”