Justice is a major concept throughout the book of Proverbs. It should also be a major concern of any society that wants to endure. When a people know that there is justice, there is peace and stability. When people begin to suspect that justice is lost - there is unrest, discontent, and even rioting. Therefore, the process of justice, which always involves witnesses, is vitally important.
The "rascally witness" is actually a worthless man who has no qualms about distorting his testimony or lying on the witness stand. The word here is "belial" and it refers to someone who is worthless because they have no moral fiber. They are good for nothing, are often seen as perjurers, and are among those who live in constant rebellion to the king or the government. This makes them unfit to serve as a witness to anything other than their own wickedness and sin. When this kind of man is on the stand - there is a complete mockery of justice. This godless man mocks and derides the entire idea of justice. He will make his own justice and his own judgments - often with no reference to justice or true righteousness.
We are also told that the mouth of the wicked spreads iniquity. The actual word for "spreads" here means to swallow something or engulf it. What the wicked man does is swallow iniquity. He enjoys sin and iniquity like the average person does delicious dish made for him. He considers sin and evil something to be consumed - not something to be avoided. when there is wickedness to swallow down - he swallows it wholeheartedly.
One other thing about this lying, false witness is his name throughout history. Here we have the phrase "a witness of belial," meaning a worthless witness - but it also means a witness of the devil. World history has had many of the "sons of belial" and many of his witnesses as well. Muhammed, Joseph Smith, Karl Marrx, and Adoph Hitler are only a few of these sons of belial. They have borne false witness of God - and have led many thousands into religious and political error. But there is one last son of Belial that will top all the others. In Revelation 13 we see the ultimate "false witness" rise up looking like a lamb - yet speaking like a dragon. He speaks powerful lies and deceives the world promoting the Beast and the Antichrist in the last days. He is the ultimate liar - will mock justice in a way that none have done in all history - and will spread iniquity, sin, misery, and death like no other. This is why a title like "witness of belial" is one we should shrink back in horror at receiving. It is also why we want to be known as people of truth - and ultimately people of THE truth of the gospel. Then we will not be known for spreading iniquity, but rather spreading righteousness. We will spread the ultimate story of God's justice at the cross - and the life that it gives to those who receive it.