How we cite our quotes:
what other bond [do we need]
Than secret Romans, that have spoke the word,
And will not palter? (2.1.15)
Does Brutus really believe everyone is as honorable as he is just because they're all Romans? How does he view Cassius's motives, and why then should he distrust Caesar?
Decius, well urged. I think it is not meet
Mark Antony, so well beloved of Caesar,
Should outlive Caesar. We shall find of him
A shrewd contriver; and you know his means,
If he improve them, may well stretch so far
As to annoy us all, which to prevent,
Let Antony and Caesar fall together. (2.1.9)
By suggesting that they also murder Antony, Cassius removes the veil of honor from the plan. This isn't simply about protecting Rome from tyranny; it's making practical moves to protect them from further "annoyances" in whatever it is they plan after the murder.
Believe me for mine honor, and have
respect to mine honor, that you may believe. Censure me in your
wisdom, and awake your senses, that you may the better judge. (3.2.2)
Brutus still believes that he retains his honor, even after the whole "murdering our leader" incident. It's interesting that he relies on his honor to convince the people the murder was justified, when it's likely that the murder is the very thing that compromised his honor. That Brutus doesn't see this is probably a good indicator that he actually did have honorable intentions: he intended no wrong, and thus can't see how anyone would think so.