1)
Why does it say "l'Makeh Mitzrayim bi'Vchoreihem"?
1.
Rashi: It does not say Bechoreihem, rather, "bi'Vchoreihem". The Bechoros cut the legs of their fathers, who were blocking Bnei Yisrael [from leaving], when they heard about the 10th Makeh (Makas Bechoros) from Moshe.
2.
Radak: It mentions Makas Bechoros, since via it, Yisrael left [Egypt]. Malbim - this was the evil that came to Egypt for the sake of benefiting Yisrael.
