There are two main differences between this 10th plague and the nine that preceded it that I wish to focus on. First of all, in all of the other plagues, the Israelites were spared automatically. For this one, though, they are given explicit instructions as to what they must do to be spared. They have to prepare a sacrificial lamb, smear the blood on their door posts and lintels, and eat the sacrifice together with matza and bitter herbs. Further, they must eat it in a state of readiness to depart, with staff in hand.
The second difference is that this plague is preceded by a number of seemingly tangential commandments, especially the one about tefillin, or, phylacteries. These are small boxes containing chapters from the Torah that are tied onto the arm and the forehead during prayer services.
Why these differences? Why is it so critical for the Israelites to perform this sacrificial ritual, and what is the relevance of the commandment of tefillin?
God does not engage in punishment as vengeance. When there is divine punishment, it is didactic. It is to teach man to correct his ways. Jewish tradition teaches that God's punishments are "measure for measure," meaning that they directly address the sinful attitude of the transgressor. This is no more apparent than in the most famous of Biblical dictums on Justice: an eye for an eye, a tooth for a tooth, a leg for a leg, and so forth.
In many parts of the world, this principle is interpreted literally, and that is a tragedy. Jewish law states that what is required by this dictum is monetary compensation, not the amputation of the limb of the aggressor. Simple justice requires this understanding! Let's say, for example, that a singer cut off the hand of a pianist. Is justice served by cutting off the hand of the singer? The pianist has lost his livelihood, while the singer can continue unharmed! That is not justice.
But there is a more fundamental understanding of this dictum that we must learn. It is that punishment is not vengeance. Those who interpret this literally make a tragic mistake, understanding punishment as a form of doing to the aggressor what he did to the victim. He caused the victim great pain, so he should suffer accordingly. What this understanding says is that the past is what matters. A. caused B. to suffer in the immediate past, so he must suffer as well.
That is not what the Torah teaches. When we evaluate the monetary worth of the severed limb, and require the perpetrator to pay that, we are stressing two things. First of all, that the future is the most important element. The aggressor has denied the victim their capabilities for the future, and it is that which must be compensated. Secondly, it focuses the aggressor on what he has done to the victim. He must think of how he has changed that person's life. We have suddenly moved our focus from the perpetrator himself to the other, to the victim.
Now we can explain the plague of the firstborn in greater depth. Egypt was a completely self-absorbed nation. It was the home of all manner of sexual licentiousness, and a pagan belief system that elevated material possessions and wealth and power. It was a society that lived for pleasure, that lived for today. This goes in direct opposition to human nature.
Human nature is built upon planting trees, upon beautifying the world for others, for the future. Our spirit drives us to sacrifice our rest and sloth to go out and build something for posterity. For this, our children are not only recipients, but transmitters into the continuing future. We are not to teach them to pursue pleasure, we are to teach them to pursue responsibility and posterity.
The Egyptians taught their children something else. They taught them hedonism, materialism, pursuit of fleeting pleasures and possessions. God brought the 10th plague, the slaying of the firstborn, as a lesson to the Egyptians: by living your lives according to your false values, you have spiritually killed your own children.
We see in the world around us tragic examples of child destruction in a spiritual sense. How can we watch the Isis terrorists teaching young children to kill without our feeling deep sadness and revulsion? And if those children get killed in some future battle, who are the real killers? I would say the grown-ups who trained them to walk down that tragic path.
Thus, the Israelites must prepare for this plague as well by reaffirming their commitment to the future. They are to eat the Passover sacrifice in readiness to march, staff in hand. This entire ritual is to cause them to embrace the future, to commit to transmitting their spiritual heritage to their children, their children's children, and beyond, into eternity. By doing this, they save their firstborn spiritually, and they are spared the plague of the Egyptians.
What is the legacy they are to transmit? In one word, Torah. God's Word is the spiritual life of the universe. The tefillin, with the four chapters of Torah within them, must be tied upon our arms and our heads. They must guide our actions and our thoughts. They are the keys to the future, they are the keys to giving spiritual life to our children. The answers to all of life's mysteries and questions can be found in the Torah, but only by delving deep into its secrets. This commitment is what gives us life, and what gives us true meaning.