Most likely because Popillius wants JC dead, too. You have to remember that the eight conspirators weren't the only people who felt that Caesar was getting out of control. There were many people who agreed that Caesar needed to be taken down (maybe not necessarily killed, but certainly in need of being reminded why Rome was so great).
Another example of this is the actions of the crowd after the assassination. There certainly were people who crazily left the senate yelling and screaming in terror, but if there weren't a large number of supporters for the eight men in attendance, they would probably have been killed in retaliation. Popillius's lines to the conspirators is Shakespeare's way of showing us that they weren't alone in their thoughts.