In Things Fall Apart, how is Ibo society to blame for Umuofia's collapse?
This is an odd question about Things Fall Apart. There seems to be nothing in Achebe's text that would indicate he intends any blame to be attributed to Ibo society itself. In addition, by his own statements, Achebe wrote the novel to show the non-African world that tribal Africa didn't have a long history of ravaging savagery as Western ideology interpreted African tribal life to have been.
The blame that the text assigns for the "collapse" of Umuofia is attributed to the British missionaries and government officials who--without knowing, understanding, or respecting anything of the deep and perhaps to Western eyes hidden moral and spiritual Ibo laws--disregarded the ways and laws of the Igbo's land and superimposed on Umuofia their ways, motives, and laws bred from ideological and philosophical precepts and propositions diametrically opposed to those of the Ibo society.
Achebe spends all of Part I and Part II drawing a detailed and authentic picture of Ibo life, thought, spirituality, and moral and spiritual laws. Then in Part III he dashes this vivid picture of Ibo life up against the brick wall of British colonization that has divided a previously unified society and bred enmity and animosity. In Chapter 20 of Part III, Achebe makes it clear that the only option, which was resistance, would end up in total devastation and Umuofia would be wiped out the way Abame was already wiped out.
If any blame within the society Umuofia can be found within the text of the novel, it is that the Ibo converts to Christianity abandoned the Ibo moral principles of unity and of bringing the other person forward to advantage while you yourself gain advantages. Ironically, it is also painfully true that in abandoning such Ibo principles, they also violated (as the missionaries themselves were doing) the highest Christian laws of love and of putting others before yourself, Christian laws kindred with Ibo laws.