Antony has decided that he will use the murder of Caesar to gain power, even after agreeing with all the conspirators that they must work together, according to Brutus' impassioned speech. In fact he vows in his soliloquy to start a horrible civil war that will tear Rome apart.
Cassius is between Brutus and Antony and sees somewhat more clearly the way that Antony's plan of having Brutus speak at the funeral will not turn out well for Brutus or the other conspirators. He knows that making sure to mark themselves as the ones who killed Caesar may not be the best idea in the end.
On the other end of the spectrum is Brutus. He believes so strongly in his own innocence and the purity of their motives that he cannot suspect Antony of anything nor does he believe that things will not work out for the best so he urges them to bathe their hands in the blood to mark themselves as the conspirators for everyone to see.