If there is a difference between "really appalling, dreadful behavior" and evil, how would you explain it?

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.