In context, Zossima is saying that if we are upset or appalled by the actions of others, we should look at ourselves and our own behavior. If we do not live our own lives as an example, we've done nothing to avert the problems at the root of the evil. We should count ourselves as a part of the reason for others following the wrong path, if we haven't led them down the good path by our example.

Accept that suffering and bear it and your heart will find comfort, and you will understand that you too are guilty, for you might have been a light to the evil-doers, even as the one man sinless, and you were not a light to them. If you had been a light, you would have lightened the path for others too, and the evil-doer might perhaps have been saved by your light from his sin.