Considering the Bible states 'you will reap what you sow' and at the time Hebrews were practically ones who didn't practice human sacrifice for the God they believed in. . .Egypt on the other hand. . .the pharaoh's 'administration' were expected to sacrifice themselves to the gods when the boss died.
So when God was making a mockery of the Egyptian pantheon via the plagues, the Egyptian way of human sacrificing got mocked hard. Also the Pharoah kept lying and going back on his word. . .AND BREAKING THE PROMISE(S) THAT PREVIOUS PHARAOHS MADE AND KEPT CONCERNING EGYPT, HEBREWS, AND GOD! Every time in the Bible when someone or a group who made a literal promise or oath to God, the following condition is always put in by God - "If either party/person of the promise/oath breaks the promise/oath then something monumentally bad will happen to the party/person."
All those Pharaohs up to the one who went up against God and Moses kept the oaths and promises. That last one broke the promises and oaths left and right an thus the 'monumentally bad thing' happened to Egypt which caused everyone their firstborn. And we can't blame God on because He DID NOT break his part of the oath/promise.
