The pain of the slavery and afflictions meted out by the Mitzri'im to the Jews actually made them better Jews. Through the afflictions put upon them, the poison of the nachash which had always cleaved to Israel was drawn out by the Mitzri'im leaving Israel pure, sanctified and elevated to a state where they were worthy to receive the Torah from Hashem on Mount Sinai. This high spiritual state could not be achieved without the afflictions and harsh labor which the Mitzri'im forced Israel to endure in their land known as the "iron furnace."

The Maharam Alshich z"tl, writes at length in Parshas Shmos that the reason for Israel's exile into Mitzrayim was to have them endure the travail of slavery because it benefited their spiritual well-being by having to endure the slaughter of their children and their physical bodies becoming the mortar of the edifices they erected. The Midyanites, on the other hand, poisoned Israel by making them transgress Torah laws. In Hashem's eyes, the actions of the Midyanites was far worse than those of the Mitzri'im and therefore their punishment is likewise great; although Israel is charged with smiting them that is not sufficient, Hashem commanded that they are to be forever beaten down.

A parable brought in the Sefer Dvir relates a conversation which occured between a snake and a leech.

Snake: You, leech, are kindred to me for we both bite mankind. Yet why does humanity treat us so differently ? People purchase you for silver and lovingly bring you into their homes, however when people find snakes, they smite their heads and flee their presence.

Leech: Although both snakes and leeches bite people, we are unrelated. When we bite people the poison that resides in them is removed and brought into our own system, however when snakes bite people, they place their own poison into the systems of who they bite.

Although the Mitzri'im harshly and severely bit the flesh of Israel, this bite drew the poison out of their collective system. The Alshich writes that the poison that resided in Israel was not able to be removed completely from Israel until they suffered extreme afflictions at the hands of the Mitzri'im. Mitzrayim was therefore called a furnace because a furnace perfects the gold and silver which passes through it by removing all its residual impurities. Therefore comparing Mitzrayim to a leech is wholly appropriate. The Midyanites on the other hand are the snake because they poisoned Israel by subjecting them to Pe'or and acts of immorality and moral turpitude.