Plagiarize means to use someone else's words and appropriate them for their own purposes.  In that sense, the Book of Mormon borrows heavily from the Bible.

To make itself appear more legitimate, the Book of Mormon lifts Bible passages to make itself appear more "true" and to align itself with the same God.  The Qur'an does the same exact thing.

2 Nephi is the worst offender in the Bible.  From Chapter 12 to Chapter 24, it requotes Isaiah Chapter 2 to Chapter 14, practically word for word.  Joseph Smith, in 2 Nephi 11:2, quotes Nephi as saying [allow me to] "write more of the words of Isaiah, for my soul delighteth in his words. For I will liken his words unto my people, and I will send forth unto all my children."  He equates himself and his mission with Isaiah, one of the greatest prophets.  This too is dangerous.

Why couldn't God quote new verses to Nephi, comforting his soul with words just for him?  Is God limited in the number of things he can say, so he has to borrow from himself?  Look at how many prophet books (Isaiah to Malachi) God communicated never asking one prophet to re-use the words of another prophet.

Nephi was supposedly written 599-545 BC and Isaiah was written around 740-680 BC?  God couldn't think of anything new to say in nearly 200 years?

Clearly, the words of the Bible were used to make the Book of Mormon appear more legitimate and truthful.