Could you please tell me who is responsible for Okonkwo's death in "Things Fall Apart"? Is it his father or someone else?

Okonkwo, like all great tragic heroes, is largely responsible for his own death.  He was out of the area when the missionaries came in because he'd been exiled for killing a man.  That exile meant that he didn't have time to slowly adjust to this new world he found when he came home.  However, that exile was his own fault.  And even more importantly, when he was faced with these white men who were threatening his sense of power and manhood, he reacted as he always did--in anger and out of a fear of being thought of as "less than a man."  So, this insecurity and anger led him to commit the act for which the white man plans to imprison him.

And this is why Okonkwo is truly responsible for his own death... he killed himself.  He committed one of the greatest sins in his culture or the white culture.  His tribesmen couldn't even touch his body when they saw him hanging there.

So, Okonkwo can certainly blame his father for instilling a certain feeling of inadequacy, but it was Okonsko himself who turned that inadequacy into anger, murder, and eventually suicide.