Killing Trees using Copper Nails?

It's faster, safer and more reliable to simply use a saw or even an axe.
If you have to ask about such dubious methods I strongly suspect you want to use it on a tree you don't own. Something I would gladly report to the police if I ever saw someone doing it.

 

I wouldn't condone using such methods if you don't own the tree and it's not on your land.

I've not tried copper nails, not even sure that works, but I have drilled and killed some wild grown suckers trees that were very out of hand on my allotment.
I I didn't cut them down as I couldn't guarantee that they wouldn't take out a couple of garden fences on the way down.
That was more than ten years ago and they have slowly disintegrated over the years and the stumps have now rotted out.
But I agree if it's not your tree don't do it.