We evolved to want to believe that we have free will because the alternative (that our actions are predestined), would lead to fatalism and a lack of the motivation needed for us (and for our genes) to succeed.  In a similar vein, simplifying concepts, such as perfect love and true evil, help us to analyse and codify the world around us (where a nuanced view might prevent us from making any decisions).

There is no reason to believe in concepts such as Free Will and True Evil, no evidence to support them, other than that they fit with our evolved intuitions.  Although nothing is certain, that they do not exist is the most pragmatic view, and so this is what we should believe.