According to Tom Cherrington, one should take special notice of the order and meaning of Nephi's words in 1 Nephi 1:19-20. We begin with the following:
"And it came to pass that the Jews did mock him because of the things which he testified of them, for he truly testified of their wickedness and their abominations[.]"
The readers should notice that the above quotation has been ended with a "period" rather than the semicolon which is in the present edition. This is because the term "mocking" is an action which corresponds to someone telling someone else that they are wicked. Mocking someone is not the same as trying to kill someone, which is the response of the Jews in verse 20 ("they sought his life"). So what caused the Jews to want to kill Lehi? Let's move on with the text:
And [next] he [Lehi] testified that the things which he saw and heard, and also the things which he read in the book, manifested plainly of  the coming of the Messiah, and also  the redemption of the world. (1 Nephi 1:19)
Now notice the increase in agitation of the Jews:
And when the Jews heard these things [(1) the coming of the Messiah, and (2) the redemption of the world] they were angry with him [Lehi]; yea, even as with the prophets of old, whom they had cast out, and stoned, and slain; and they also sought his [Lehi's] life, that they might take it away. (1 Nephi 1:20).
Thus the anger of the Jews against Lehi progressed from mocking to seeking his life. But what did Lehi say that so angered the people of Jerusalem that they sought to take his life? There must have been something in his testimony that the Jews felt was so offensive that they deemed Lehi to have been guilty of a crime for which death was the proper penalty. Apparently it was his testimony of the coming Messiah and the redemption of the world, for this not only caused the Jews to seek Lehi's life, but also the lives of "the prophets of old." But why was their message so bad?
What if Lehi had actually declared that the Messiah who should come, even Jesus Christ was Jehovah? Could the Jewish beliefs of the day accept that Jehovah would be made flesh? And what would be the reaction of those people if they were to be told that they would put Jehovah to death? We first might consider the consequences of preaching more than one God, that Christ was the Son of God. The record of Jeremiah, a contemporary prophet during Lehi's time in Jerusalem (see 1 Nephi 1:4; 7:14) would seem to indicate that the people of Jerusalem were certainly not totally devoted to the worship of Jehovah. He repeatedly declares that the people of Judah have "forsaken" the worship of Jehovah (See Jeremiah 1:16; 2:13; 2:27; 5:12; 7:17-18; 7:31; 9:1-6; and etc.) Jeremiah will go so far as to say, "A conspiracy is found among the men of Judah, and among the inhabitants of Jerusalem. They are turned back to the iniquities of their forefathers, which refused to hear my (Jehovah's) words; and they went after other gods to serve them: . . ." (Jeremiah 11:9-10). Therefore, that Lehi may have reminded them that there was a second member of the Godhead was not likely the cause of their desire to kill Lehi. The people of Jerusalem were more likely offended by Lehi's message that Jehovah would become mortal and die. Perhaps this is why the wicked priests of Noah felt justified in sentencing Abinadi to death hundreds of years later (see Mosiah 17:7-8).
When Lehi testified of "the redemption of the world" to the people of Jerusalem, could this have been considered to be a blasphemous teaching worthy of death? It would have been impossible to have taught the Jews concerning this "redemption," without testifying that the prophesied Messiah of the Jews would in fact be killed by them.
The term "Lamb of God" is used 33 times in the Book of Mormon. Of these, Nephi, or an angel instructing Nephi, uses the term 30 times. The term "lamb" referring to a savior figure is only used once in the Old Testament. Christ is referred to as the Lamb, or Lamb of God, in 29 verses in the New Testament, of which only one does not have reference to the mission of Jesus Christ. Approximately 25 of these verses are used by John the Revelator either in his gospel, or the Revelation.
It is interesting that in nearly all of the verses that Nephi uses this term, he is relating what he saw, or was told in his Tree of Life vision. This is significant since Nephi had asked to see, hear, and know what his father had learned in his Tree of Life vision (1 Nephi 10:17). Apparently John the Revelator was shown much, if not all, of the same vision:
And behold, the things which this apostle of the Lamb shall write are many things which thou hast seen; and behold, the remainder shalt thou see. . . And I, Nephi, heard and bear record, that the name of the apostle of the Lamb was John, according to the word of the angel. (1 Nephi 14:24-27)
Surely, the frequent usage of this term by Nephi would indicate that the sacrificial nature of the Savior had been emphasized by him in his teachings. If Nephi actually adopted the use of this term from his father's teachings or their shared visions, this would indicate that Lehi may have emphasized the sacrificial nature of the life of the prophesied Messiah. And thus perhaps it was this testimony that the Jews would kill their Messiah that caused them to utterly reject Lehi (and the prophets of old).
While we may never know, the exact reason or reasons for the Jews' desire to seek the life of Lehi, we can be certain that they rejected his message concerning "the Messiah" Jesus Christ. Consider the words of Nephi, the son of Helaman, to the apostate people of Zarahemla, approximately six centuries later:
Yea, did [Moses] not bear record that the Son of God should come? And as he lifted up the brazen serpent in the wilderness, even so shall he be lifted up who should come. . . .
Yea, and behold I say unto you, that Abraham not only knew of these things, but there were many before the days of Abraham who were called by the order of God; yea, even after the order of his Son; and this that it should be shown unto the people, a great many thousand years before his coming, that even redemption should come unto them.
And now I would that ye should know, that even since the days of Abraham there have been many prophets that have testified these things; yea, behold, the prophet Zenos did testify boldly; for the which he was slain.
And behold, also Zenock, and also Ezias, and also Isaiah, and Jeremiah,. . .
Our father Lehi was driven out of Jerusalem because he testified of these things. Nephi also testified of these things, and also almost all of our fathers, even down to this time; yea, they have testified of the coming of Christ, and have looked forward, and have rejoiced in his day which is to come. (Helaman 8:14-22).
It is this message of Jesus Christ that was so difficult for the people of Jerusalem to accept in Lehi. Whether it was a monotheist verses polytheist conflict, whether it was the simple fact that Lehi testified that they would kill their Messiah, or whether it was that Jehovah would become mortal, Lehi's testimony of Jesus Christ was the focal point of his conflict with the people of Jerusalem. For this, the people of Jerusalem sought his life. Could these same questions concerning "the coming of Jesus Christ" be at the foundation of nearly all of the disputes, rebellions, and wars among the Nephites and their enemies for centuries to come? [Tom Cherrington, "Why Was Lehi Forced to Flee from Jerusalem?," unpublished] [See the commentary on 1 Nephi 2:1, 10:5, 10:10, 12:18; 2 Nephi 25:19; Mosiah 3:8; Alma 7:10; Alma 13:16; Helaman 8:19-20]