It has to be said - it's a nice juxtoposition. The slaughtering of the lambs occurring as they slaughter Jesus. The lambs themselves symbolising the sacrifice YHWH asked the Israelites make before smearing the blood over the door so that the righteous could be identified and they would be spared God's wrath.
Jesus represents the sacrifice of the lamb which represents the means to avoiding YHWH's wrath which is the path to salvation. That John chose to have the two events occur at the same time is symbolism, used to get the reader to draw the two stories together and to think of Jesus in a certain fashion. The actual lambs saved the Israelites, the symbolic lamb (Jesus) saves the world.
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