More killing?

Exodus 12:12 says, “For I will pass through the land of Egypt this night, and will smite all the firstborn in the land of Egypt, both man and beast; and against all the gods of Egypt I will execute judgment: I am the LORD.

12:13 And the blood shall be to you for a token upon the houses where ye are: and when I see the blood, I will pass over you, and the plague shall not be upon you to destroy you, when I smite the land of Egypt.”  I don’t understand why god could separate the people during the swarms of flies (Exodus 8:22)  but he can’t during his bloodlust of killing all of the firstborn men and beasts.  Why kill the beasts at all let alone the sheep killed for the blood swatches on the Hebrew homes.