"And the L-rd killed every first-born … therefore, I sacrifice to the L-rd every male first-born, etc.", wherefore our sages have said: The first-born of beasts are slaughtered in consideration of the first-born of beasts (that were killed in Egypt), and the first-born of men are redeemed in consideration of the first-born of men (i.e., the Israelites, that were saved in Egypt.)