Okonkwo was upset with his son Nwoye for not meeting his expectations of being strong, masculine, and ambitious like himself. He viewed Nwoye as weak and unmanly, which was a source of great disappointment for him.
Subjective answer: The thing that upsets Okonkwo the most internally, is the fact that Nwoye is too much like his grandfather Unoka. The one action that gets Okonkwo the most upset externally is finding out that Nwoye has been hanging around the Christians, at which point Okonkwo confronts Nwoye and nearly chokes him to death.
Because it is his son's duty. Okonkwo is the master of the house.
"Okonkwo was a man." "Did you see that man, Okonkwo?" "You are my son. Okonkwo, I am your father!"
Okonkwo's son.
At first Ikemefuna is fearful, being away from his mother and father, but in time he adapts, becoming an older brother to Nwoye, and becoming Okonkwo's son. Ikemefuna even calls Okonkwo 'father'. He becomes the most manlike of the children in Okonkwo's eyes.
Okonkwo is Unoka's son. Okonkwo does not respect Unoka, as Unoka is looked down upon by the villagers.
Ikemefuna called Okonkwo father. It would be unnatural for a father to kill his son. The boy was also affectionate towards Okonkwo, and it would be like a betrayal.
Ikemefuna is the boy that is sent to live with Okonkwo. Eventually the boy becomes much like Okonkwo's own son, and Ikemefuna even calls Okonkwo father.
Their maternal instincts appear, and they feel sorrow for the loss of one who they treated as their son only a day before. Further, Okonkwo is in a poor mood afterward, and they must deal with him.
Okonkwo is killing off the part of his family that he is the most proud of. He wishes not to become weak like his father, yet he cuts down the part of his family that is most like an ideal son.
Obierika suggested that Okonkwo should let him take care of the crops until Okonkwo could repay him. He also proposed that Okonkwo should leave his son Nwoye with him as a pledge to ensure Okonkwo's return.
It is not specifically mentioned in the book. Okonkwo puts most of his focus on Nwoye, since he is the eldest son, but Nwoye does not really meet his standards for being a man. Okonkwo seems to respect and like his adopted son Ikemefuna more, even though Ikemefuna is not truly his son. Later, his focus is mostly on his daughter Ezinma, whom he continually wishes had been born a boy.