Many people accept Jesus but many ask why did he have to die?

BECAUSE only the blood of JESUS the one and true lam of GOD could wash away the stench of sin even when HE knew many would hate and reject HIM.thank YOU JESUS for shedding YOUR blood for the  forgiveness of my sin's.

The first human, Adam, was created perfect, without sin. However, he chose to disobey God. Adam’s disobedience, or sin, profoundly affected all his descendants. “Through the disobedience of the one man,” the Bible explains, “many were made sinners.”​—Romans 5:​19.

 Although Adam was created to live forever, his sin brought upon him the penalty of death. Through Adam, “sin entered into the world and death through sin, and so death spread to all men because they had all sinned.”​—Romans 5:​12.In contrast, Jesus’ death not only removed the blemish of sin but also canceled the death sentence for all who exercise faith in him. The Bible sums up matters this way: “Just as sin ruled as king with death, so also undeserved kindness might rule as king through righteousness leading to everlasting life through Jesus Christ our Lord.”​—Romans 5:​21. Of course, humans today still have a limited life span. However, God promises that he will grant righteous humans everlasting life and resurrect the dead so that they too can benefit from Jesus’ sacrificial death.​—Psalm 37:29; 1 Corinthians 15:22.

Perhaps "many" have not read Isaiah 53 and 1 Cor. 15:1-4.

He didn't "have to" die. God doesn't "have to" do anything. But He wanted to make very clear both the horrific nature of sin, and the extent of His love for us.

Many more use correct grammar:  Many people accept Jesus but many ask why he HAD to die.  "Why did he have to die" is a sentence in itself.

Those that accept Jesus know precisely why Jesus had to die. He died willingly to pay the penalty for our sins.

Since we are talking about fiction, why did Captain Ahab have to die?

To bring in the New Creation, the old being ruined by sin.

The Bible answers that Q, starting in Genesis 3:15, then in prophesies about Him coming in Isaiah 52:13 to 53:12. Some of those verses read, "He was wounded for our transgressions, he was bruised for our iniquities. The chastisement of our peace was upon him, and with his stripes we are healed... the Lord hath laid on him the iniquity of us all... thou shalt make his soul an offering for sin... my righteous servant shall justify many, for he shall bear their iniquities... because he hath poured out his soul unto death; and he was numbered with the transgressors, and he bare the sin of many, and made intercession for the transgressors." 

He had to die in order to do the only thing that would deal with our sin - which we can never atone for ourselves. Him dying was the fulfilment of Genesis 3:15, the seed of the woman being bruised in the heel. But now the risen Christ is crushing Satan in the head, and the rest of the Bible explains that. People need to study the Bible to understand why Jesus had to die, and HAD to be resurrected - as proof that He truly is the sinless Son of God, who alone can be "the Lamb of God that takes away the sin of the world" (John 1:29).

King James Version

Matthew 20:28 Even as the Son of man came not to be ministered unto, but to minister, and to give his life a ransom for many.

John the Baptizer stated that Jesus “takes away the sin of the world.” (John 1:29) This drew attention to Jesus’ role in saving obedient mankind.