To speak against others, that is, to speak of another’s faults, is disrespectful and causes disharmony. Listening to such talk can poison our mind against another.
This is the second of four precepts drawing attention to speech. When we speak, our speech has consequences for others and for ourself. A tendency to fault-find, whether in our thoughts or in our speech, fuels mistrust and prevents us from seeing the Buddha Nature in all beings.
How much more compassionate it is, and how liberating, both for oneself and for others, to cultivate kindly speech and to look for another’s virtues instead of their shortcomings, to rejoice in their merits.
The sixth Great Precept does not, by the way, mean that we should cover up wrong doing. Speaking out about harmful behaviour and injustice in order to prevent suffering is quite different from speaking of another’s faults out of insecurity or spite.