The word evil can have many different meanings, such as morally bad or wrong, causing ruin, pain or injury, or an evil force, power, or personification. Simply by the definition of evil, one can only have a grasp of what evil is, but only through experience one can understand evil fully. Evil is readily perceived differently among people of certain, religions, races, ages, sexes, and mental prowess, but the underlying factor of evil is always geared toward negative outcomes, either physical or moral. Evil is when one purposefully causes pain, not pain caused by fault. Evil is knowing something is morally wrong, but still proceeds in doing so. A great misunderstanding about evil is that unintentional harm to some extent is not evil at all.
For example if you " re driving along a street and you ran over a cat accidentally and killed it, that is not evil. Although being evil is unadmireable, it is necessary because without it there will be no good. How can one define good, without evil. Just as how can you have love without hate. It is just like yin and yang, evil balances out the good in this world. Although I am not glorying evilness, there is always some evil in everybody.
Where do we draw the line between evil and good? We can't. The boundary is based on one's opinions, usually dependent on religion, childhood, and mental prowess. Religious people might think stomping on a roach is so evil that you will go to hell, or people with a deprived childhood might think shooting dogs with homemade bolt guns is fun, like in the movie "The Good Son." Basically evil is based on each individual.