In his speech, he says "Believe me for mine honor, and have respect to mine honor, that you may believe". He is using his creditability against the fault of him killing Caesar.
Brutus also says "Not that I loved Caesar less, but that I loved Rome more. Had you rather Caesar living and die all slaves than that Caesar dead, to live all freemen." He is trying to keep people from rebelling so he is trying to say that Caesar dying was better for Rome.
He also says " As Caesar loved me, I weep for him; as he was fortunate; I rejoice at it; as he was violent, I honor him: but, as he was ambitious, I slew him." He is trying to get people to believe his honor. He is trying to prove that Caesar was ambitious and he would treat Rome badly if he was crowned.