The concept is 'made clear' to you, since the time you are able to understand the world.
When you are still in your half-pants and frocks, you are taught 'paap' and 'punyam'. The difference between God and Asura is described in detail. You are made to understand (over a number of years), what (certain) things are 'forbidden'.
As you grow older, you venture out into the world. You come across people, form opinions and absorb experiences .Your childhood definition of Evil changes .You modify it to suit your own needs.
You understand that lying is not 'Evil', it is necessary. Stealing is also okay to some extent, and so is 'talking bad' about people. Most of us stop here, and turn out to be 'civilized people’.
Some other unfortunate ones go ahead and become murderers , terrorists and rapists.
The point being, Evil is a relative term ! It changes it's boundaries and definitions (much like it's propagators, the form changing jinns and rakshasaas)
WHO DECIDES AS TO WHAT IS 'EVIL' ?
Who decides as to whether you committed a sin ? Who do you look up to , for relieving your burden of guilt ?
Here's a secret , no one . Your priest, your inspirational writer, your guru ,your parents and your friends are all on the same level as you.
All of them get confused and scratch their heads at some point or the other .They all rely on the same instrument as you (eventually) do , the omnipresent 'conscience' .
That inner instinct that beeps every time you step out of line .For it carries the wisdom of ages within it .It remembers all that you have been ,since the beginning of time.
And sometimes ( when it suits us) , we choose to mute it completely.
THE REAL POWER
The definition of evil has changed its forms, from the mythical demons to Hitler and Osama. Evil has changed uniforms and methods. But, what has remained constant is the firm belief in 'Evil'
What if I told you that there existed no Evil. That it is just a fragment of your imagination. In the end , it's your conscience that decides what is evil . You; and you alone decide if your actions and thoughts are 'sinful' or not.
You would explode; all of us would. It's a great power, to be free of the burden of judgement. To be able to do anything, as long as your mind claimed that it was okay.
Ponder over it, and reflect . Maybe that's why all religions lay down a million definitions of Evil. They are scared of making man realize his inner power. They bind man with fear, guilt and shame.
And rightly so, for man is weak and unstable .If left untamed, we shall stop at nothing.