What is evil? Like most terms in the english language that are not tangible objects (persons, places, things) evil is an abstract concept. Meaning is attributed to the word as a result of values and social interactions of the members of a society. The meaning of the term is socially constructed, and all words referring to abstract concepts have only the meaning given by society. Furthermore, those meanings can morph based on the context in which the word is used.
Justice Stewart said of obscenity in Miller " I know it when I see it." Can the same not be said for evil? We can't actually see evil, but we can see evil acts. Such acts are the type which society has deemed so heinous, so morally reprehensible that their perpetrators must not be allowed to co-exist with the rest of society. No one has compiled a complete list of "evil acts" but yet we recognize such acts because of their deviation from those actions which society considers "normal."
The people who commit these actions do not see them as "evil." The Manson's, Daumer's etc. have explanations for their actions. To the rest of us they seem twisted beyond belief, but the men committing these evils don't see their actions in the same way. Despite the fact that society considers their crimes "evil" these men have justified their actions.
The fact is that Will Smith's comments are not ridiculous. Those who are bashing his words are oversimplifying what he is saying.