The fallen nature of man is seldom properly explained by Christianity which is why it makes little sense to many people eg there is a seeming contradiction or illogicality in God creating people who are seemingly so fatally flawed. And how can this alleged 'original sin' apply to me when I don't even know what it is that I'm supposed to have done !?
However, there is at least one way of understanding this 'fatal flaw' so that it can begin to make sense.
If we recognise the nature of God as selfless (as is the nature of love) and recognise the nature of man as self-based (as is the nature of selfishness) it is possible to see that firstly there is a gigantic divide between us and God and secondly that selfishness and humanity are inextricably linked.
The good news is (and this is the whole purpose of religion) is that man can return to the selfless state (heaven).