EGO: What is it exactly?

Given how hard the Gnostics pound the elimination of the ego as an essential key to salvation (for lack of a better term), I think this is a question worth asking.

I am quite new to Gnosis, so forgive me for being a newbie to these concepts. The introductory teachings I have studied so far say that every human being is a multiplicity of egos or "pyschological Is" and they are the primary cause of our apparently chaotic behaviour. These egos seem to be treated as ethereal, sentient entities that feed off of our ethereal energy. Why do they do this? Is it simply because it is their nature or do they have some malicious intent towards humanity? They apparently explain the "mental and emotional schizophrenia" found in most human beings, but can't our internal and external behaviours simply be explained as the aggregated results of pre-programmed responses to past environmental stimuli? Is it really necessary to bring what amounts to "possession" into the picture? How do we know that egos actually exist?