I think that the critical portion of the question is the last part. I am not sure that Achebe is able to fully voice his support for either side. It is evident that he believes that the imposition of Western culture on the indigenous people of Africa contained elements of disaster within it. The breaking of cultural bonds as well as the forced nature of a new order upon indigenous cultures were disastrous. At the same time, I don't think that Achebe is glorifying indigenous cultures that were not receptive to change, and unwilling to adapt. Achebe does not romanticize indigenous cultures as he details their propensity for tribal sanctioned violence and silencing of voice.
In the end, Achebe might be making a larger statement about the nature of change. Change seems to be a permanent part of one's being. Achebe voices support for those who are able to successfully navigate change while remaining true to their identity:
...while deploring the imperialists' brutality and condescension, [Achebe] seems to suggest that change is inevitable and wise men ... reconcile themselves to accommodating change. It is the diehards ... who resist and are destroyed in the process.
The "diehards" exist on both sides. They are the forces that Achebe sees as the reasons for the suffering of many. In seeking to criticize the "diehards" who do not engage in the reflection needed to navigate change, Achebe makes clear where his support lies.