Balancing self-interest and society benefit

Self-interest is one of the most accused words and ideas in mankind's history. It has been blamed for every disaster political , economical, environmental etc. On the other side the tendency to benefit society and other individuals has been considered divine-related, a higher quality. The first is considered to be the source of evil while the latter the source of good. In my opinion these viewpoints are outdated. The definition itself of good and evil is an overgeneralisation that was made to set rules to the mass and define its behavior to support a normal society function. It has been used also for the interest of special groups. Good and evil are easy to understand even for a small child but is it not that something good for me can be bad, harming for another and vice-versa? Can we really define them?
In my opinion by his nature human is a being that works toward his own interest. It is the way things work in the living creatures. The self-preservation and improvement of level of life is the target of every person. Towards that goal many species have discovered the benefits of cooperation creating societies. In societies the benefit that is offered from a member to the group is usually returned back by improving the level of life of each member beyond the individual's ability. For that reason humans but also other species have evolved to need the communication and contact of a group on intellectual and emotional level but also to have the tendency to offer their services to society and other individuals, to help. This evolved so high that an individual can feel pain when the other persons is in pain for example,resulting to compassion.
The cooperative tendency is opposite to the selfish tendency. They are the two ends of the line of human behavior. There are many styles of people with behaviors standing on some point of this line. What is the best area of this line?
Some will tell that people must be cooperative and compassionate and even self-sacrificing in order the society to improve and reach its goals. But this is against principle number 1 of existence that is preserve your life and improve it. Every time someone sucrificies his own interest and stands on the cooperative, offering side of the line, someone will take advantage of this in order to satisfy his own interest. It is in human nature. There can be no selfish people with total ignorance about society rules when there are not people to stoically sit and watch their rights diminish. The person that really deserves to be the example of the succesful society member is the one standing near the middle of the line. This person does what he must do to achieve his personal goals and satisfy his interest but he puts some limits to his actions concerning how harmful they can be for others or the society. He will do something opposite to another person's interest but within boundaries. This person will help other people too and offer benefit to the society but he will not let people step over him and go against his rights. This person offers to the society and himself almost equally (there is space for diversity). If we train a society of such people that can assess a situation and the pros and cons for theirselves and the others and then act in a balanced way we can create a balanced society that will not tolerate selfish people without boundaries and force them to either balance themselves or be left outside. There will be no victims of their goodness and no exploiters because each individual will be in a balanced state of acting to fullfil self-interest and society benefit being at the same time complete individuals but also succesful parts of the society.
What I claim in this post is that the way to improving ourselves and society is through balancing our goals and thinking the consequences in both short and long terms (self-interest is generally more short-term than the benefit from offering to the society) of our actions. We are to try the best for us and try it in the least harmful way for the others. Each one of us can find his or hers balance and be consciously selfish and offering to the others in a suitable proportion.