Brutus thinks that honest men will follow through on their intentions; therefore, they do not need to swear an oath.
In Roman culture, you simply were true to your word. No oaths were needed, and this is the opinion the Brutus holds to.
What exactly are you asking?
Thoreau believed that sometimes the government is dysfunctional. He believes that the government would often abuse their power. He also does not believe in the idea "majority rules." He thinks that the minority are the real leaders. He thinks that the minority is going against the majority because the minority can see the loopholes of the government which others can not see.