The missionaries are one of the most destructive forces to enter the plot, yet you can really argue this both for good and for bad. The problem is that while we as readers despise them for trying to change the Igbo culture that by that point in the novel we have become immersed in, we are also relieved that they are trying to change some of the more offensive aspects of that culture. For instance, Achebe does not hide the strong patriarchy or the murder of twins, both of which offend Western sensibilities.
At the same time, we don't want to see all aspects of the culture disappear. For instance, social mobility is extensive in the Igbo culture, and we are sad to see the clan meetings broken apart. Justice is also fair, and the new rules don't allow for the traditional punishments.