Why did Moshe take blood that was already sprinkled on the Mizbe'ach to sprinkle on Aharon and his sons?
Chizkuni: In order to connect them with the Mizbe'ach. 1
The Da'as Zekenim and Hadar Zekenim left this difficult, seeing as, after Zerikah, the Mitzvah pertaining to the blood is finished, and the Kedushah is gone - if it spilled on a garment, it need not be washed (Zevachim 93a) PF.