as for example cancer or certain kinds of psychopaths
Well in Cancer's situation, which is a thing and thus can't be evil anyway, its redeeming factor could be that it isn't trying to hurt you it is just following its own programming that got messed up.
In a Psychopath's situation they are people who are driven crazy who likely can't tell what is really going on or cannot understand the nature of the situation (though there are psychopaths whos insanity did lead them to commit a crime but who who have no psychosis that really prevented it.), their redeeming factor is usually that even though they are insane they are attempting to do good, however limited it may be, within the scope of their madness.
-Also Psychopaths arn't nessisarily murderers I believe.
closest you'll come to perfect goodness are certain altruistic humans but all such admit to being flawed
There is an old saying: "Even a Thief will go out of his way to help a friend"
So if you looking for pure evil people you are not going to find them either. Sociopaths, according to some psychologists, are people who took in the wrong hints and messages society sends out at a regular basis and simply follows them to their logical, and often destructive, conclusion.
Look the only point that in Dwarf Fortress Evil seems to be a force. Curses exist, cursed lands exist (as well as their mechanics to become cursed), and even creatures who are evil exist. Yet good is fanciful and just as destructive and dangerous to all life. There is no benevolence only malevolence.
which is what is essentially the question
Where is the Benevolence?