One of the most often asked questions regarding Parshas Vayeira is how G-d could interfere with Pharaoh's free will and yet punish him. Or so that is what appeared to have happened to many people.
The Parsha states that G-d hardened or alternately strengthened Pharaoh's heart. The heart is the seat of emotion, differing emotions. Pharaoh and his people could have had one of two possible responses to G-d's message and his plagues.
One of course was fear, and indeed we see in this Parsha that some of Pharaoh's servants experience the fear of G-d's word and direct their slaves and animals indoors. Pharaoh himself experiences fear at times and accedes to Moshe to some degree.
The second would be love. Odd as this may seem. The plagues initially began as wonders that did not kill but served to warn and shock. Like the burning bush Moshe saw, they contained the message of the supremacy of G-d which Pharaoh had denied, over man, animal, water, sky and earth. More than that they contained the justice and mercy of G-d, a justice that punished evil and a mercy that showed forbearance to evil even permitting it numerous chances to repent.
Strengthening the heart for Pharaoh induced courage that held off fear. A strong heart is a courageous heart. But a strong heart is also one that is capable of love. Strengthening Pharaoh's heart did not prevent him from repenting, it simply gave him the courage so that he would not surrender out of fear and yet empowered him to repent out of love.
At each turn of the road the Egyptians had chosen the evil path. Rather than learning from the famine that G-d is the ultimate master of existence, they turned all the more strongly to idol worship. Rather than learning from the Jews who were among them, they enslaved and oppressed them. Rather than learning from G-d's warnings, they persisted in their ways.
As a man raised among Egyptian nobility who was at once descended from a noble Jewish line, Moshe was the perfect prophet to convey Egypt's last warning, quietly but firmly. And when love failed and fear prevailed, the ending had already been written.