And even then, the usual punishment for high-status citizens (before and through the lifetime of Jesus) was exile. Even in the chaotic days of the late Republic, the Roman state didn't execute Romans for treason, not technically. They would normally put such citizens on lists of proscribed people whose assets were forfeit to the state. Anyone could enforce the proscription by killing the citizen and handing over his assets to the state for a reward.

That's how Cicero met his end, for example. Pompey Magnus also died at the hands of private people at the behest of the Roman state, though he has just landed in Egypt seeking refuge.