1. Question Restatement:
Why was Elijah so afraid of Jezebel that he fled immediately after performing a great miracle?
Quick Answer: Elijah was afraid because he had just realized that even though God's heart was to reconcile people to him, Elijah had instead turned reconciliation into an execution - just "like his fathers". Why wouldn't anyone be terrified once they've realized that there was never any possibility for them to justify death - especially when they could have instilled life instead?
1 Kings 19:4 - ... for I am not better than my fathers. ... 12 and after the earthquake a fire, but the Lord was not in the fire; and after the fire a still small voice.
2. An Apparent Contradiction - Was Elijah Actually Afraid to Die, or Not?
Afraid to Die?
NASB, 1 Kings 18:3 - And he was afraid and arose and ran for his life and came to Beersheba, which belongs to Judah, and left his servant there.
Or, Not Afraid to Die?
NASB, 1 Kings 19:4 - But he himself went a day’s journey into the wilderness, and came and sat down under a juniper tree; and he requested for himself that he might die, and said, “It is enough; now, O Lord, take my life, for I am not better than my fathers.”
So, Elijah's fear couldn't have just been about "dying".
3. Something Jezebel Said Changed Elijah's Life:
Elijah had JUST outran Ahab on the way TO Jezebel who was in Jezreel:
NASB, 1 Kings 18:46 - Then the hand of the Lord was on Elijah, and he girded up his loins and outran Ahab to Jezreel, (where Ahab's palace was, 1 Kings 21:1).
Elijah was certainly not afraid of Jezebel, but was afraid of the judgment she had pronounced ...
Jezebel Reminded Elijah of what he JUST did to the prophets of Baal:
NASB, 1 Kings 19:2 - Then Jezebel sent a messenger to Elijah, saying, “So may the gods [lit. "God", (Elohim)] do to me and even more, if I do not make your life as the life of one of them by tomorrow about this time.”
Elijah had a realization that Jezebel was justified to invoke "God", (Elohim), to affirm a judgment against him - and so he ran.
But, What Had Elijah Just Realized?
NASB, 1 Kings 19:4 - But he himself went a day’s journey into the wilderness, and came and sat down under a juniper tree; and he requested for himself that he might die, and said, “It is enough; now, O Lord, take my life, for I am not better than my fathers ... 12 and after the earthquake a fire, but the Lord was not in the fire; and after the fire a still small voice.”
4. Elijah was afraid for his life, and judgment for what he did - by God:
Although I am sure that Elijah felt perfectly justified when he executed 450 people, God actually showed up to change peoples' hearts to repent - not to execute anyone.
In this same way - today, theists misrepresent God by exchanging the desperate love and mercy of God for a vengeful and wrathful God, (which God actually is in a way); but, God is vengeful towards those who subvert the desperate and unconditional love and mercy of God. Unbelievers, (like Jezebel against Elijah; and even the "donkey" against Balaam) are perfectly justified in their judgments. So, why wouldn't anyone be terrified?
But, even Jesus gave up trying to explain this, saying instead:
NASB, Matthew 9:13 - But go [away] and learn what this means: ‘I desire mercy and not sacrifice.’ For I did not come to call the righteous, but sinners, to repentance.”
Elijah and "his fathers" had condemned themselves by rejecting mercy:
Elijah, and "his fathers", completely missed the heart of God - they had the wrong spirit the entire time. Elijah finally realized that he, himself, desperately needed to find repentance; but alone, he did not have the strength to pursue repentance and withstand the shame; he would likely continue to stumble and moreover be judged by his own judgments - with no mercy:
Elijah, like his fathers, subverted how God desired to prove himself:
NKJV, 1 Kings 18:24 - Then you call on the name of your gods, and I will call on the name of the Lord; and the God who answers by fire, He is God. ... 1 Kings 19:12 ... but the Lord was not in the fire; ... a still small voice.”
NASB, Ezekiel 7:27 - The king will mourn, the prince will be clothed with horror, and the hands of the people of the land will tremble. According to their conduct I will deal with them, and by their judgments I will judge them. And - they will know - that I am the Lord.’”
Elijah had only just understood that he had had the wrong heart, and was mortally grieved by it - finally understanding that God desired mercy and not sacrifice (punishment):
NASB, Ezekiel 18:23-25 - Do I have any pleasure at all that the wicked should die?” says the Lord God, “and not that he should turn from his ways and live? ... 25 “Yet you say, ‘The way of the Lord is not fair.’ Hear now, O house of Israel, is it not My way which is fair, and your ways which are not fair?
New Testament Clarification on What Elijah Did:
This entire argument presupposes the validity of Jesus' claim that God had all along desired mercy, and not sacrifice, (not punishment) - and that it was necessary for the Christ to die to show that only by trusting in (his) unconditional advocacy (mercy) brings life.
A person is only justified to appeal for mercy, if they plead for the mercy of others; and so, a person cannot justify pleading under the law of mercy, unless they have also forfeit the right to condemn.
It requires a fundamental change of mind to see that this is THE constant theme in ALL of Scripture. From mercy: Eve's advocacy for Adam - saving his life, Job being healed once he prayed for his friends, Esau embracing his brother, a harlot defending terrorists, (literally), to condemnation: Samson's last "noble" act of killing thousands; Israel's "righteous" annihilation of men, women, and children; and on and on and on.
NASB, Luke 9:54 - When His disciples James and John saw this, they said, “Lord, do You want us to command fire to come down from heaven and consume them?” 55 But He turned and rebuked them, and said, “You do not know what kind of spirit you are of; 56 for the Son of Man did not come to destroy men’s lives, but to save them.” And they went on to another village.
NASB, Luke 24:44 - 45 And He opened their understanding, that they might comprehend the Scriptures. 46 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, 47 and that repentance and remission of sins should be preached in His name to all nations, beginning at Jerusalem.
NASB, James 2:13 - For judgment is without mercy to the one who has shown no mercy. Mercy triumphs over judgment.
Conclusion: That fear, that Elijah found, is the beginning of wisdom - and why he ran. It is a terrible fear and burden to seek forgiveness when you realize that there was never any possible justification to cause death, especially when you could have instilled life instead.