As so often the proverb is elliptical and only makes sense in its unstated context. It refers to a specific situation without mentioning all the details, and leaves the reader to work out what that situation is.
If we are tempted to do others harm, let us fear the God who has unlimited power to turn our own hand back against us. Let us take care to always act towards others, even those who have deliberately harmed us, with justice and not with overreactions based on hatred or revenge.