What an intriguing question! I might equate the difference between "really appalling, dreadful behavior" and evil with difference between something really sad and something tragic. It's a matter of degree.
Even the worst behavior can be considered less than evil as long as the motive is not evil. When both the bad behavior and the motive are intentional, there is often a cruelty involved which cause it to rise to the level of evil. As to degree, if the bad behavior is wreaked on one, it's sad and wasteful and horrible. If that same behavior is unleashed on many, it becomes evil.
In spiritual terms, evil--not just bad or awful--is the enemy of, or opposite of, good.
the tree of knowledge of good and evil in the purest non judgmental form ...in the beginning man did not know good or evil --because neither existed until a thought arose ,something ingested that shouldnt have been that made division inside of man. a belief that people are divided. equality erased, and differences noted. this is the beginning of mans judgment on eachother. the truth is that good and evil are imaginary. i mean they exist but only because of the other and only because of the belief in one or the other. if you combine a postive with a negative you get a cross which symbolizes the end of conflict and the rise of equality and unity and the number zero which is neutral. true non judgmentalism is the knowledge that we are all the same. its it an ongoing struggle but i hope it can be achieved.