Despite the love and respect Brutus had for Caesar, he decided to take part in the conspiration against him and assassinated the emperor himself. In Brutus's words in his speech in the Capitol, we can read "not that I loved Caesar less, but that I loved Rome more" as an explanation for his bloody deed. Since Caesar's ambition was dangerous for the Romans, Brutus slew him.
I did a 3 second google search for "tim o'brien" and the first thing that came up was the novel he wrote about the vietnam war called "The Things They Carried" written in 1990.
The Abame clan gets torn. It happened because most of the clan converted to Christianity and followed the white men, but some of them are stuck to their own traditions and refuse to follow the white men. Especially Okonkwo does not want to convert.