@natedawg119 I think it helps to keep in mind the context in which these events are occurring. The Egyptians slaughtered the Israelite children and Moses’ sister saved him from that event (Shiphrah and Puah saved many more), so we are not dealing with an innocent nation. This nation had the blood of God’s people on its hands.
In addition, God did not start with the killing of the firstborn. First, God asked nicely for Pharaoh to let His people go. God even provided a miracle that should have served as a warning to Pharaoh (the staff turning into a snake and the leprous hand). God then progressively demonstrated His power over the pagan gods, who did not exist, and yet Pharaoh still persisted in holding God’s people captive and persecuting them. Only at the very end did God resort to the slaying of the firstborn.
The Egyptians killed the Israelite children in order to ensure their future enslavement. God killed the firstborn of the Egyptians in order to deliver His firstborn from bondage. There is a historical context we should not forget. It’s not as if God just randomly decided one day to slaughter the Egyptian firstborn.
What is more, you asked what would happen to an Israelite who did not put the blood over their door. But let’s ask a different question - what would happen to an Egyptian who did? I guarantee you God would spare anyone who showed faith and trust in Him. God is merciful.
Christ grant you wisdom