The quote instructs individuals to not look at humanity with naieve eyes. An integral part of the human being, evil co-exists with good. There is a daily struggle, according to the religius beliefs of Christians, between good and evil. As long as humanity consistently chooses to do good for and to each other, it will always be one step ahead of evil in the world.
In The Crucible, the townspeople recognized the presence of evil in their community, they were convinced that witches were at work in Salem. There was evil there, however, it was not due to witches, it was the evil that resides in the dark passions, jealousy, envy, anger, hate, that is present in each human being.
In a way, the people of Salem denied that evil existed within the ordinary human person, and because of it, they tried to eliminate it, something that is not possible is you believe what the quote says.
Rather than accept that evil was present in their community, within each person they chose instead to blame the Devil, believing that by casting him out, in the body of the innocent people executed, that the evil would be gone from Salem.
Another excellent example of the fact that "evil" is part of our nature, and that denial of one is denial of the other is found in Hawthorne's Young Goodman Brown. In this tale, Brown encounters (or at least he thinks he does) many of his friends from his town engaged in "evil" acts in a "Black Mass" in the forest. Of course, with Hawthorn's ever present ambiguity, it is unclear whether any of this actually happens or whether it is a dream, but the consequences are devastating. Brown can no longer accept the people (the Minister, his catechism teacher, and his wife, Faith) for what they are, mixtures of good and evil, and spends the rest of his life in isolation, dying a miserable man.
At some point in our lives, we all have to face that fact that people we thought were "better" are just people. They're not evil, they're just people.
My interpretation of this quote would be that man is naturally evil. To deny evil is to deny we are human with free thought and free will. There are two schools of thought on evil: One is that evil is natural to human beings. We are evil, but society keeps us in check. An example to support this would be the novel Lord of the Flies. Golding suggests through the character of Jack that without rules and regulations man will go while and follow the id part of his psyche. An example that might go against this is Shakespeare's Macbeth, which suggests man is basically good until he is tempted by evil. In the beginning of the play, Macbeth is a noble soldier. It is isn't until he meets with the witches, and his wife encourages him to kill Duncan that he becomes evil. So you would refute this quote by saying it is not "human" to be evil. Humans are naturally good, like Macbeth. We have the capacity for evil, but are not necessarily evil at birth.