Both the making of 'Nehushtan' by Moses (NUM 21:4-9) & its destruction by Hezekiah (2K18:4) lend themselves to colourful preaching in conjunction with our given JN passage. The incident of the snake on the pole is burned into Jesus' psyche. It is a central factor in the way he sees his life & death. 14/15 & then 16/17 are like a Psalm, with 16/17 re-stating the theme declared in 14/15. It's important to take all this as one. To focus only on JN 3: 16 as Christians often do, detaching it from its context within 14-17, is to lessen its scope & its impact!
The whole passage can also be seen as a refrain to God's loving action in NUM, &, in our case, in Jesus' Passion. God so loved the people - despite their lack of attitude - that he gave them a snake .... not in the grass but on a pole so that if they chose to, they could look to it (whatever primitive, mysterious meaning this incident bears!) & be made whole. Making people whole is what God does best - healing our various serpent bites as we choose to look to Jesus lifted on the cross & raised in triumph over that death & all death. What is it that's biting us today? Where are we to look for healing & wholeness?
Some 'brands' of faith give the impression God is, on (im)balance a condemnatory figure looking for excuse to be destructive. But as17+ helps us understand, the God revealed in Jesus is not like that at all. The very opposite. If we are condemned it will be because we choose to condemn ourselves because we don't choose God. If we are destroyed it will be because we choose to destroy ourselves. When all the time, all God wants is our wholeness. Hence Jesus, his lifting up, his making those who will look to him whole. I long ago gave up trying to 'theologise' about how the atonement works. There comes a point beyond which doing that doesn't get one nearly as far as entering into the mysteriousness of God & God's ways. These days I happily settle with Charles Wesley for 'tis mystery (& mercy!) all'!
The light v darkness contrast is another way of reinforcing what it is God offers us, contrasted with what we're prepared to accept. The bigger I make myself, the more shadow I cast on others to black God's love out from them - if that were in itself possible. If we humble ourselves as Jesus does in becoming our whole-making Light, no-one needs choose to live in the darkness of evil & the pain it brings. Yet, even in deepest darkness, God is closer than breathing......