Chemicals to kill tree roots are herbicides. The caution with that, however, is that the tree can pass the herbicide to another tree connected to it. This releases the chemicals back into the environment damaging and killing other vegetation. For the best methods in using herbicides, visit: forestry.about.com/od/silviculture/p/kill_tree_real.htm. There are ways to kill trees without chemicals.
. For more information on that, visit: forestry.about.com/od/treephysiology/a/kill_tree_organ.htm.