Round Up only works when sprayed on the foliage. Copper nails take forever, and do not work on some trees.
The quickest way to intentionally kill a tree [besides cutting it down] is to girdle it. Girdling is removing the bark all the way around the tree about 3-5 inches in height. The cambium layer exposed to the elements soon dries out, and the tree will no longer be able to send water to the canopy.
The only living tissue in a tree trunk is the cambium layer. The center of the tree is as dead as a door nail.