Human nature is essentially good. They have many elements to their nature and each one has the potential to bring them benefits.
A sin is committed when someone causes harm to themselves or to others or to any part of creation. The guilt depends on the intention of the sinner. In its most extreme form someone does deliberately harmful and destructive acts rejecting any appeals to do what is for their own benefit never mind what is beneficial for others. They may claim that it makes no difference anyway since existence is pointless and therefore have no gratitude for the benefits they have in life.
The contrast to this is someone who tries to improve himself, others and all of creation. They believe in God and are always grateful to Him for all they have in life. Their works to improve creation flow from their will to please God.
Why does the perfect God allow sin?
Human beings have the capacity to sin largely as a result of having the capacity to plan. When someone plans their efforts, they need to be able to suppress their natural desires for a time. This is quite different from animals who live from moment to moment obeying their perceptions of the present and their instinctive drives. This is indeed a dramatic difference. Human beings are able to look to the future - conceptualise it and form an intention to act. This conscious intention can override even the most powerful of our instincts. Through it we have capacity to cause ourselves harm in the short term in order to realise the greater good in the long term. As an inevitable part of this we gain the potential to cause ourselves harm, i.e. the potential to sin.
We cannot see clearly into the future. What we do instead is to believe in some future circumstances and direct our actions accordingly. Taking planning to its logical limits we would try to do what is for the good over all time and certainly for our entire life (in this world and the next). This is the core of trying to do what is morally right.
In attempts to understand what the future will bring, human beings have searched for ever better explanations for what they encounter of reality. This search is part of morally good thinking. The fact that we can never fully know the future is analogous to the fact that we can never really know God. God is something in the 'unseen' part of reality, as is the future.