Most translations of Exodus 12:21 render Moses's words to the elders as a simple instruction: go and take a lamb. Targum Pseudo-Jonathan sharpens it into a rebuke. "Withdraw your hands from the idols of the Mizraee," Moses says, "and take to you from the offspring of the flock, according to your houses, and kill the paschal lamb."
According to this reading, on the very night of the Exodus, the elders of Israel still had their hands on Egyptian idols. Four centuries in Mizraim had left a theological residue that had not been fully scrubbed by the nine plagues. Moses is not congratulating the elders on their faithfulness. He is confronting them with a choice that must be made before the redemption can begin.
The rabbis often treated this verse as the hinge between slavery and freedom. The physical Exodus requires a spiritual Exodus. To kill the lamb — the sacred animal of Egyptian religion — is to publicly renounce those gods. To tie one to the doorpost for four days and then slaughter it at twilight is to complete the renunciation.
This is why the command goes through the elders. The old guard, the ones with standing, are the hardest to turn. Moses reaches them first because without their public break, the rest of the people will hesitate. When the elders put down the idols, the nation will follow.
Takeaway: Israel did not walk out of Egypt clean. Moses made the elders wash their hands before the journey began.