When Moses came to Pharaoh, he declared plainly what would happen and every single word of Moses happened. It was a war and they were well warned.
Exo 4:22  And thou shalt say unto Pharaoh, Thus saith the LORD, Israel is my son, even my firstborn: 
Exo 4:23  And I say unto thee, Let my son go, that he may serve me: and if thou refuse to let him go, behold, I will slay thy son, even thy firstborn.

Pharoah and the whole of Egypt only spent time in increasing the oppression and trying to duplicate anything that Moses did. Even when his magicians were saying this is the hand of God, Pharaoh kept playing games with Moses and deceiving Moses to stop the plagues. Many times, Moses told them plainly and forewarned them on what would happen. Very few of them complied. Was it only Pharaoh that perished at the Red Sea, no.

Exo 9:19  Send therefore now, and gather thy cattle, and all that thou hast in the field; for upon every man and beast which shall be found in the field, and shall not be brought home, the hail shall come down upon them, and they shall die. 
Exo 9:20  He that feared the word of the LORD among the servants of Pharaoh made his servants and his cattle flee into the houses: 
Exo 9:21  And he that regarded not the word of the LORD left his servants and his cattle in the field. 

Moses knew most of them and said this very plainly about them.

Exo 9:30  But as for thee and thy servants, I know that ye will not yet fear the LORD God. 
Exo 9:34  And when Pharaoh saw that the rain and the hail and the thunders were ceased, he sinned yet more, and hardened his heart, he and his servants. 

When God was to execute the judgement of the firstborns, He still gave conditions for anyone and everyone to meet for salvation. Some mixed multitude Egyptians obeyed and were spared. Other incorrigible Egyptians faced the judgement headlong and still followed to the Red Sea like mad people.

Exo 12:7  And they shall take of the blood, and strike it on the two side posts and on the upper door post of the houses, wherein they shall eat it.
Exo 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. 

We still have similar people of mad character among us daring God. The blood of Jesus is available for all as usual. After judgement comes, someone will open thread in hell asking why.

lasthero:
Just wondering.
The king pharaoh is the one who didn't allow the Isrealites to go, not the fathers of the first born, not even the innocent first borns.


Why didn't God deal with only pharaoh and his family alone?, why did he use the entire firstborn of innocent fathers to settle scores?

Romans 9:10 And not only this; but when Rebecca also had conceived by one, even by our father Isaac; 9:11 (For the children being not yet born, neither having done any good or evil, that the purpose of God according to election might stand, not of works, but of him that calleth 9:12 It was said unto her, The elder shall serve the younger. 9:13 As it is written, Jacob have I loved, but Esau have I hated. 9:14 What shall we say then? Is there unrighteousness with God? God forbid. 9:15 For he saith to Moses, I will have mercy on whom I will have mercy, and I will have compassion on whom I will have compassion. 9:16 So then it is not of him that willeth, nor of him that runneth, but of God that sheweth mercy. 9:17 For the scripture saith unto Pharaoh, Even for this same purpose have I raised thee up, that I might shew my power in thee, and that my name might be declared throughout all the earth. 9:18 Therefore hath he mercy on whom he will have mercy, and whom he will he hardeneth. 9:19 Thou wilt say then unto me, Why doth he yet find fault? For who hath resisted his will? 9:20 Nay but, O man, who art thou that repliest against God? Shall the thing formed say to him that formed it, Why hast thou made me thus? 9:21 Hath not the potter power over the clay, of the same lump to make one vessel unto honour, Andrew an inother unto dishonour? 9:22 What if God, willing to shew his wrath, and to make his power known, endured with much longsuffering the vessels of wrath fitted to destruction: 9:23 And that he might make known the riches of his glory on the vessels of mercy, which he had afore prepared unto glory,




Genesis 15:13 And he said unto Abram, Know of a surety that thy seed shall be a stranger in a land that is not theirs, and shall serve them; and they shall afflict them four hundred years; 15:14 And also that nation, whom they shall serve, will I judge: and afterward shall they come out with great substance.



Purpose means reason for creation. Once a creation serves the reason for creation, that creation has fulfilled purpose.

It is the creator of a creation that decides it's purpose , and no one should begrudge the creator for making the creation serves it's purpose.

It would be foolish of a person to complain over the usage of dustbin for collecting waste because that it was it was made for.

Reading the above scriptures, you will clearly see that it was the destiny of pharoah and the Egyptians to suffer and did the way there did. That is what they were created for. That was thier purpose, it is the height of stupidity to question a creator over the purpose of his creation.

Straight to the question. Biblically, when God speaks to a person 'singular' He does not speak to him as an entity but a nation or generation.
If God has told you He has blessed you, He doesn't mean you as an individual alone but to those you represent either as a father, head of an Organization, governor of a state or President of a country. Same as curse.
Question: Who is Israel? Jacob..! And God called Jacob Israel in Gen 35:9-10 from an entity to a nation. So everytime God will refer to Jacob, he refer to him as Israel (A nation not as an individual) 1Pet 2:9.

Going back to Egypt
When God told Pharaoh to let Israel go, He was not talking to singular Pharaoh but to the entire Egypt whom Pharaoh represented as king. So God was talking 'Nation to Nation'.....
So, the death of the first born of Egypt including Pharoah' s son because God was dealing with Pharaoh as a nation not as a king.

kingxsamz:
■ The question is "what was the offence of the firstborn of Egypt?" wetin dem do? Not only Pharaoh's first born was killed. Every first born of the Egyptians were killed. And this was after the 10 plagues. So most of the families of Egypt must have wanted the Israelites to go so they can live in peace. All these explanations you gave has nothing to do with the question asked.

21 While Moses was traveling back to Egypt, the Lord spoke to him, “When you talk to Pharaoh remember to show him all the miracles that I have given you the power to do. But I will cause Pharaoh to be very stubborn. He will not let the people go.
22 Then you should say to Pharaoh, ‘This is what the Lord says: Israel is my firstborn son.
23 And I am telling you to let my son go and worship me. If you refuse to let Israel go, then I will kill your firstborn son.’” - Exodus 4 vs 21 - 23

A firstborn son typically refers to one individual in a family but in this case, the term is used in reference to a whole nation of people, a first indication that it is meant to be figuratively processed. In that passage, anyone able to see could tell that God described the people of Israel as having the same importance that a firstborn son would have to the likes of Pharaoh whose firstborn son He, God, threatened.