The only way that this quote applies in Shakespeare's Macbeth (I think) is that a few good men may be unable to take action. There is no question at the time of Duncan's murder, when Macduff discovers the King's body, that there is a committed group of ethical (sans Macbeth) and loving subjects that are determined to find out who is responsible for taking the life of their beloved King.
The difficulty for them is not a lack of conviction, but a lack of information. Without knowing for certain who is responsible, there is no way for these men to act. It is, then, in this situation that evil can thrive, for Duncan's subjects—Macbeth's peers—do not have anything to act upon other than conjecture and rumor (specifically, that Malcolm and Donalbain are responsible—in truth, they flee Scotland because they fear for their own lives, for they, too, have no idea from where the danger emanates.)
For a thesis statement, which I assume deals with this scenario, I would write something similar to the following:
Evil often appears to have a life of its own, seemingly impervious to attempts to stop it; however, what is worse than the evil is a lack of action by good men or women to take steps to stop it—for if a few good individuals fail to take a stand against it, evil will flourish.
The nature of evil is such that it perpetuates where good people fail to stand against it. There will always be evil. What overcomes evil is actively standing against it. The play MacBeth is an excellent example of evil that triumphs for the lack of action be good people. Both the characters and events succumb to the forces of evil because there are no good forces to actively stand against them.