I don't know whether I have share this example of a word of knowledge that was not positive for the person receiving it. It concerned a church elder who came forward for prayer and the preacher had a word of knowledge that the married elder was in an adulterous relationship. The preacher knew that if he had blurted it out in front of everyone it would have destroyed the elder and severely damaged the church. So he asked the Lord, "How do I share this word of knowledge without causing a nuclear explosion?" The Lord replied, "The bare facts of the word is for you. Now I will give you a word of wisdom on how to share it. Tell the elder that ensuring faithfulness to your family will continue My blessing on his life." The preacher shared the word of wisdom the Lord gave him and the elder broke down and confessed his adulterous affair. As a result, the elder was successfully counselled, his marriage was saved, the stability of the church was preserved, and although the elder had to stand down from eldership until his domestic situation was stabilised, he eventually returned to ministry and the blessing of God continued in his life.
So, I agree that a word of knowledge can be negative, and can reveal sin, but along with the word of knowledge there must be a word of wisdom for how it should be shared. Often a word of knowledge can be the basis of a prophetic word, and so the word of wisdom is needed to be able to phrase the resulting prophecy so that those responsible for the sin are convicted of the Spirit without "collateral damage" to innocent people.
Too often, negative prophecies are given without that word of wisdom, and serious harm to churches and people have resulted from "friendly fire".