It is likely that Joseph Smith (and most Christians) would associate the symbol of a serpent with the devil, as per the story of Adam and Eve in Genesis. However the symbol of a serpent was a dual symbol in the ancient world which could represent the devil or deity.
In the Book of Mormon, the symbol of the serpent is used both positively and negatively. It is portrayed positively as Christ in Helaman 8:14-15:
Yea, did he 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. And as many as should look upon that serpent should live, even so as many as should look upon the Son of God with faith, having a contrite spirit, might live, even unto that life which is eternal.
And portrayed negatively as the devil in 2 Nephi 2:17-18:
And I, Lehi, according to the things which I have read, must needs suppose that an angel of God, according to that which is written had fallen from heaven; wherefore, he became a devil, having sought that which was evil before God. And because he had fallen from heaven, and had become miserable forever, he sought also the misery of all mankind. Wherefore, he said unto Eve, yea, even that old serpent, who is the devil, who is the father of all lies, wherefore he said: Partake of the forbidden fruit, and ye shall not die, but ye shall be as God, knowing good and evil.
Why would Joseph Smith compare Christ to a serpent? How did Joseph know about this dual symbolism?