For follow-up discussion and general commentary on the topic. Comments are sorted chronologically.
Agnes Stuart
Just a small point. When Moses was asked to make a serpent for the people to look to in the wilderness it brought healing, so in this case the serpent is a positive symbol although I don't understand why it was used.
Jim Brady
Serpent represents sin (from the Garden), brass judgment (brass altar,etc because it could hold fire).
Sin was hung on a pole and judged and that 'pole' raised up dropped in the ground and sin paid for by Jesus on the cross. Just like He told Nicodemus in John chapter 3 that as Moses lifted up the serpent in the wilderness...the Son of Man must be lifted up.
Jesus death on the cross brought healing of the sin problem. So, the snake itself is not positive but the judgment of sin that is represented is.
Most likely that bronze pole would have had some type of short cross member to hang the snake on so it looked like a cross.
Hope this helps.
note: the actual brass snake & pole was destroyed in 2 Kings 18:4 because they were worshipping it.