Bo: Ohr HaChaim HaKodesh: Mitzvah 5 – Concept 404
“The entire congregation of Israel shall slaughter it towards evening.” (Exodus 12:6) We are commanded to slaughter the Pesach offering at the specified time. (Rambam, Hilchot Korban Pesach)
When the Midrash speaks about the Israelites having to divest themselves of vestiges of idol worship this is not to be understood as their being idol worshippers.
However, they still used to wear clothing that the pagans wore, and ate foods that the pagans ate. The fastest and most effective way of countering the psychological impact of their former practices was to set aside the Egyptian diety in the knowledge that they would slaughter it in a few days’ time.
Inasmuch as the Egyptians believed in astrology, the constellation of the ram represented a handicap that Israel had to overcome. Slaughtering a ram was the most effective way of doing this. (Or Hachaim Hakdosh)