When Marlow discusses Kurtz's burial, he claims the following:
The next day the pilgrims buried something in a muddy hole. (Section 3)
What is ironic about this burial?
Based on his European ancestry, Kurtz's body should have been returned to England for a proper burial. This is ironic because Kurtz was very concerned with proper appearances.
By burying Kurtz in Africa, his intended was not allowed a proper farewell. This is ironic because Kurtz's Intended believed that he would have wanted to be buried where she could visit him until her death.
Kurtz is buried in a "muddy hole," a place of emptiness and filth. Marlow calls Kurtz "something" rather than by name. This is ironic because Kurtz has been worshiped by the natives, yet he is reduced to a nameless person who is buried unceremoniously.
Marlow chooses to allow Kurtz to be buried in a "muddy hole" because he can't stand to be reminded of what Kurtz had become. This is ironic because Marlow has begun to become less civilized at this portion of the novel.
Kurtz is quickly buried in the "muddy hole" to prevent the cannibals from eating him. This is ironic because the cannibals had a healthy respect for Kurtz and would have maintained a sense of restraint following his death.