Let us excuse semantics and the definition of words; after all:
'A word is know by the compnay it keeps'
To me evil is when something that opposes your set of beliefs or morals or principles causes, or is likely to cause, unhappiness to you. Because everyone's beliefs, morals or priciples vary, there is no universal definition of the cause of evil because it is variable. Thus, the devil doesn't exist as a singular entity because, although the effect is unhappiness, the cause will vary between two parties; what is good for one thing (e.g. food 'hunter'
is bad for another (e.g. the food 'prey'
Above and beyond this mod of thinking, which is materialistically realistic, there is a whole pool of variability which underlies the effect, but which if accessed allows one to FEEL why we THINK this way.