It seems like these verses follow naturally from what preceded in the previous chapter. For anyone who sins and does not know it, it is still sin and they are guilty and they have to make atonement for it. But notice that is not how this chapter begins. This chapter begins with making sure that if you have testimony which would exculpate someone, then you are not to keep that testimony to yourself, but you are to speak out.
It is a good thing to address the sin of inaction as opposed to the sin of action. Often we focus on those things that we do wrong as the sin which we ought to remove from our lives. Interestingly enough in our first world lives it is often the things that we do not do or the words which we do not say which can cause the most harm. When we do not speak out against injustice, we do harm. When we remain silent when we can make a difference, or when we keep to ourselves our material goods and do not look for ways to reach out to those in need, then we do harm. The sin of inaction is much more insidious than the sin of action. At least in the sin of action we have something concrete to approach and address. In the sin of inaction we are on our own, we have to discover it for ourselves. I encourage you to find ways in which we are sinning by our inaction and take steps to remedy it.