Part of this proverb repeats verse 6. Some want to reverse the meaning here and render it, ‘the mouth of the wicked covers violence’, on the grounds that both parts of the verse then speak of the effect of words on others, not on the speaker.
Even as believers, if we are wayward and backslidden, everything we say, you can really tell from the colour of the rage across the mouth, what is generating it, the unsettled, or selfish, or jealous feelings that rage around in our innermost being that drive us or produce the conversation. So you can hear one person speaking and the words will be constructive and encouraging and humble and helpful and you hear another person speaking and this person is being berated, and that person is being run down. One person's speech is being driven from unsanctified feelings, the other person's speech is sanctified.