"And Egypt enslaved the children of Israel with great harshness. And they embittered their lives with hard work, with mortar and bricks and all manner of work in the field, all the work they were enslaved with was done with great harshness." (Shemos 1:13-14)
There is a well-known midrash that teachers that the word: baferech (great harshness) is a hint to how the slavery began. Although the Torah is brief and gives the impression that it started suddenly, the midrash explains that, in fact, Pharoah enslaved our ancestors a bit at a time, starting with voluntary work for the state that then became involuntary, with the burdens increasing over and over until our ancestors were completely enslaved to the Egyptians.
Rav Reuven Wabshat of Netivot notes in his book, Machaneh Reuven, that the Rambam once referred to Pharoah as the yezter hara. Why should Pharoah be considered the embodiment of our evil inclination any more than the other villians in Jewish history?
The answer he gives is that unlike those other villians, Pharoah convinced us to enslave and embitter ourselves. When the slavery began our ancestors, not realizing what they were getting themselves into, showed up willingly for work, not through force. Proof from the midrash comes from the tribe of Levi which, as the Torah implies, was not enslaved because they never showed up for work on the 1st day.
Rav Shimshon Rafael Hirsch, in his commentary on the enslavement of our ancestors notes that at the Convenant between the Pieces between God and Avraham, three characteristics of the exile in Egypt were predicted: that the Children of Israel would be strangers in a land not their own (geirus), that they would be enslaved (avdus) and they would be humbled through torment (inui). He notes that when the Torah begins to discuss the slavery, those three stages quickly come to pass. First Pharoah labels our ancestors as a strange people that the Egyptians have reason to suspect and fear. They had been in Egypt for over a century at that point and one of their ancestors had saved the country from famine while enriching it at the same time but what were they? Ivrim, not Egyptians. This was the stage of geirus. The next stage was the onset of slave labour, avdus, and finally the great harshness of the work, as described in detail by the midrash, inui.
But what does this have to do with Pharoah being the yezter hara? The metaphorical answer is that the story of our slavery in Egypt is told by the Torah not only for historical and religious purposes but also as a metaphor for our characters.
The evil inclincation is something we live with every day but as many of the giants of Mussar note, amongst them the Michtav M'Eliyahu, the evil inclination does not approach the God-fearing Jew with an open demand to sin. Such a tactic would lead to failure. Instead, it makes subtle suggestions. The Shulchan Aruch, in the very first chapter of Orach Chayim, warns against this tactic of the yetzer hara on cold, dark winter mornings when staying in bed late is so tempting. The yetzer hara doesn't tell us not to daven, but to skip the minyan and stay in our warm homes. Then comes the thought that some parts of davening can be skipped, and finally the progression to not davening at all. Each step along the way seems rational at the time, a logical next step in the turning away from a Torah lifestyle without thinking that we have abandoned it at all.
Thus was Pharoah. The midrash tells us that on the first day of the enslavement, he showed up to help out with the work, sending out the message that if Pharoah can show up and get his hands dirty, the Ivrim can too. You can be sure he didn't show up the next day but the new slaves had no such choice. So it is in our lives. Slowly, carefully, the yetzer hara convinces us a little here, cajoles us a little there, to abandon proper behaviour without thinking we have.
We thus owe it to ourselves to reflect each day on our conduct and thoughts. Are they honestly what we expect of ourselves? Can we really justify all the compromises we make that we think are so necessary? And can we improve ourselves so that we are making the most of the opportunities God gives us and the goodness he showers on us each day?