Much of the above conversation (which was very interesting, by the way) could be condensed once terms such as 'good' and 'evil' are defined. But alas, we are bound to find sharp disagreements here as well, meaning the conversation may have ultimately expanded. Still, I think defining our terms is important for coming to agreement on a standard by which to measure any presupposition concerning human nature.
Ultimately, the original poster is 100% correct in that it is in our base assumptions that we are divided. Get to a persons presuppositions in any disagreement (and reveal your own along the way) and most disagreements can be more clearly understood, if not overcome.