I would agree that Achebe does not necessarily intend to condemn or criticize tribal customs but I always go with Lawrence’s dictum, don’t trust the teller, trust the tale. The book does show serious brutality as part of tribal life. I know from some of Achebe’s comments about the book that he may feel he’s presented the culture as a working culture with much order and beauty, not a dismissable primitiveness, and that he’s presented it for all in all, good and bad, and that Western culture also has its bad, brutal elements. But I think, at the very least, the book gives people whose minds are not bound by multiculturalism and PC some pause about what Europeans confronted in Africa, a confrontation that was bound to happen eventually one way or another. Also, Okonkwo is partialy brought down by his own division over the tribal mandate that he must kill the boy who has been chosen scapegoat, a boy who has come to love him and whom he has come to look upon as a son. Also, partly as a result of this deed, Okonkwo’s own son is estranged from him and to some extent from the tribe. All these things make it seem as if this order of things is entering its twilight even before the British come. This is partly supported by the fact that members of the tribe agree to cooperate with the British, and some flee in relief to Christianity.
I haven’t read the book in a while, so my memory may be faulty, but this is how it seems to me, basically a tragedy about an inevitable doom, a doom in which Okonkwo’s own character embroils him especially. Whether Achebe wants us to see it that way is a another matter.