Wolf in Sheep's Clothing?

In response to Lauren Lemieux's question, "Do we all have a little bit of a murderer in us but for the most part we know how to control ourselves?"...

     Lauren is essentially asking a question that is closely related to one I raised in my last post; does it appear that humans are inherently good or inherently evil? Though I found my own question unanswerable, it seems that hers has a logical response. In class we have been debating whether humans have natural tendencies, and for the purpose of this question, let us assume that they do. These natural insticts would have developed when humans were first evolving from neanderthals, or rather in what many would consider "caveman times". During this period, people would have struggled to survive amongst the fierce animals that often inhabited their environments. They hunted for food, and killed their predator if they were in danger. If the theory of natural insticts withholds, this would mean that even today, if someone was in danger they would feel the impulse to kill. I believe that we all do have a "bit of a murderer in us" for the simple reason that if someone was to attack us, most of us would "naturally" defend ourselves.
     Society, in addition to biology, has also conditioned us in many ways though, and as we have discussed in class, individuals have learned to often suppress some of the things they want to do for fear of it not being socially acceptable. For instance, one hundred years ago, people would hit their children, give them very difficult chores to complete, and marry them when they were barely even adolescents. Today, children have it "easy" comparatively, for their parents usually do not do any of these things. Nevertheless, murder is obviously socially unacceptable, and therefore the majority of people do not participate in it. So, it seems that because we have the simply instinct to defend ourselves, we do all have some sense of "murderer" in us, but just suppress it either consciously or unconsciously. But, I do not believe this "murdering" instinct humans are capable of having makes us "evil". It perhaps does if we actively do it, but maybe being able to control ourselves is the complex skill humans have that makes us relatively good.  Do you believe that evil actually exists or is it simply a socially and culturally constructed concept?