We are taught in the Mishna (Pesachim 5:2) in today’s daf (Pesachim 59b) that the Korban Pesach must be slaughtered ‘lishma’ (literally ‘for its own sake’). What this means is that even if, while being slaughtered, the slaughterer intends that this animal be used for any other purpose or offering than a Korban Pesach, they have invalidated its use.
As the Gemara elsewhere (Zevachim 7b) explains, this rule is derived from the words of Parshat Bo where we are told that ‘when you come to the land that God will give you, as He promised, you must [also] keep this service. And if it will be that your children ask you מָה הָעֲבֹדָה הַזֹּאת לָכֶם – “what is this service for you?”, you must answer them זֶבַח פֶּסַח הוּא לַה’… – “It is the Pesach offering which is for God…”’ (Shemot 12:25-27), from which Chazal derive that the Pesach offering must be intended for this reason and it must be ‘for God’. From here it would seem that the necessity for the Korban Pesach to be slaughtered ‘lishma’ is not merely a technical detail, but is, in fact, a pedagogic requirement. But why should this be so?
To answer it should be noted that the sacrificing of most animals was a symbolic expression by the one who brought the sacrifice of regret, thanks, or loyalty, and the specific animal used for this purpose – though of symbolic significance – was rarely directly associated with the intentions of that sacrifice.
However, in addition to the Korban Pesach being an offering to God, it was also a theological statement by Bnei Yisrael to the Egyptians who worshipped the lamb as a god. As such, the intentions of the Korban Pesach were rooted not only in its consumption, but in its slaughter as well.
And this is why we learn the requirement of slaughtering the Korban Pesach ‘lishma’ from the words זֶבַח פֶּסַח הוּא לַה’ – “It is the Pesach offering which is for God”, because the very act of slaughtering the Korban Pesach was a bold rejection of the idolatrous practices of the Egyptian culture, and an clear expression of Bnei Yisrael’s love and loyalty to God.