John introduced Jesus as the lamb "Behold! The Lamb of God who takes away the sin of the world! (John 1:29)

The lamb he refers to was slain. It is through His death life came to us.

A covenant in the Old Testament was not just blood but required the death of the animal (representing that life is given for sin) Heb. 9:15-16: “And for this reason He is the Mediator of the new covenant, by means of death, for the redemption of the transgressions under the first covenant, that those who are called may receive the promise of the eternal inheritance. For where there is a testament, there must also of necessity be the death of the testator.”

Heb. 9:26-28: “He then would have had to suffer often since the foundation of the world; but now, once at the end of the ages, He has appeared to put away sin by the sacrifice of Himself. And as it is appointed for men to die once, but after this the judgment, so Christ was offered once to bear the sins of many. To those who eagerly wait for Him He will appear a second time, apart from sin, for salvation.”

This is why he became man:

Phil 2:8: “And being found in appearance as a man, He humbled Himself and became obedient to the point of death, even the death of the cross.”

Heb. 2:9: “But we see Jesus, who was made a little lower than the angels, for the suffering of death crowned with glory and honor, that He, by the grace of God, might taste death for everyone.”

Jesus referred to Jonah as an example of what would happen to him Luke 11:29: “and no sign will be given to it except the sign of Jonah the prophet.”

The sign was the resurrection, which means He would have to die to be alive again.

Matt. 12:40 "For as Jonah was three days and three nights in the belly of the great fish, so will the Son of Man be three days and three nights in the heart of the earth.”

Jesus spoke of his death and resurrection numerous times

Matt 16:21 “From that time Jesus began to show to His disciples that He must go to Jerusalem, and suffer many things from the elders and chief priests and scribes, and be killed, and be raised the third day.”

It was necessary for Scripture to be fulfilled.

Luke 24:6-8 "He is not here, but is risen! Remember how He spoke to you when He was still in Galilee, "saying, 'The Son of Man must be delivered into the hands of sinful men, and be crucified, and the third day rise again.'"And they remembered His words.

Jesus told them ahead of time that He would die, it was necessary, it would fulfill Scripture i.e Psalm 22 describes in detail what he would experience and he even quotes a portion of it from the cross. Specifically Psalm 22:16-17 “For dogs have surrounded Me; the congregation of the wicked has enclosed Me. They pierced My hands and My feet; I can count all My bones. They look and stare at Me.”

Luke 24:46-47 Then He said to them, "Thus it is written, and thus it was necessary for the Christ to suffer and to rise from the dead the third day, "and that repentance and remission of sins should be preached in His name to all nations, beginning at Jerusalem.”

The Scripture says it was necessary.

John 11:49-51 And one of them, Caiaphas, being high priest that year, said to them, "You know nothing at all, "nor do you consider that it is expedient for us that one man should die for the people, and not that the whole nation should perish." Now this he did not say on his own authority; but being high priest that year he prophesied that Jesus would die for the nation”

Prophecy comes from God, it was declared even to the unbelievers and scoffers what He would do by one who held the office of the high priest.

John 2:19-22 Jesus answered and said to them, "Destroy this temple, and in three days I will raise it up." Then the Jews said, "It has taken forty-six years to build this temple, and will You raise it up in three days?" But He was speaking of the temple of His body. Therefore, when He had risen from the dead, His disciples remembered that He had said this to them; and they believed the Scripture and the word which Jesus had said.”

At his trial they repeated what he said.

Matt 26:61 and said, "This fellow said, 'I am able to destroy the temple of God and to build it in three days.'"

At the cross they mocked him

Matt 27:40 and saying, "You who destroy the temple and build it in three days, save Yourself! If You are the Son of God, come down from the cross."

John 10:17-18 "Therefore My Father loves Me, because I lay down My life that I may take it again. "No one takes it from Me, but I lay it down of Myself. I have power to lay it down, and I have power to take it again. This command I have received from My Father."

I Jn 3:16 “By this we know love, because He laid down His life for us. And we also ought to lay down our lives for the brethren.”

Heb. 2:14: “Inasmuch then as the children have partaken of flesh and blood, He Himself likewise shared in the same, that through death He might destroy him who had the power of death, that is, the devil”

Heb 10:10 “By that will we have been sanctified through the offering of the body of Jesus Christ once for all.

Col 1:21-22: “yet now He has reconciled in the body of His flesh through death, to present you holy, and blameless, and above reproach in His sight”

This is what Paul says is the gospel-1 Cor 15:3-4 “For I delivered to you first of all that which I also received: that Christ died for our sins according to the Scriptures, and that He was buried, and that He rose again the third day according to the Scriptures,

Notice it says died for our sins, not just bled, and He rose again- which means one must die to rise again. And it was told in the Scriptures. By whom, God through the prophets.

Paul repeats this as a fundamental teaching in his writings. Rom 5:6-8 For when we were still without strength, in due time Christ died for the ungodly. For scarcely for a righteous man will one die; yet perhaps for a good man someone would even dare to die. But God demonstrates His own love toward us, in that while we were still sinners, Christ died for us.”

Rom 14:9: “For to this end Christ died and rose and lived again, that He might be Lord of both the dead and the living.”

2 Cor. 5:15 and He died for all, that those who live should live no longer for themselves, but for Him who died for them and rose again.

Paul even goes as far as saying that keeping the law would not help anyone because they must receive righteousness through Jesus Christ:

Gal. 2:21: “I do not set aside the grace of God; for if righteousness comes through the law, then Christ died in vain.”

It was promised that He would be raised from the dead: Acts 2:29-32: "Men and brethren, let me speak freely to you of the patriarch David, that he is both dead and buried, and his tomb is with us to this day. "Therefore, being a prophet, and knowing that God had sworn with an oath to him that of the fruit of his body, according to the flesh, He would raise up the Christ to sit on his throne, "he, foreseeing this, spoke concerning the resurrection of the Christ, that His soul was not left in Hades, nor did His flesh see corruption. "This Jesus God has raised up, of which we are all witnesses.”

So Jesus blood was shed for the forgiveness of sin and his death was necessary to prove that his blood was accepted as the means to cleanse our sin.

“was raised because of our justification”(Rom. 4:25)

1 Peter 1:3: God “has begotten us again to a living hope through the resurrection of Jesus Christ from the dead”