"But it is illegal to execute a Roman citizen without a full trial."
The Romans prided themselves on their laws and justice, as well as on the privileges and superiority of being a Roman citizen. To execute a Roman citizen without trial would be a truly despicable crime:
“‘But I was born a citizen,’ Paul replied. Those who were about to question him withdrew immediately. The commander himself was alarmed when he realised that he had put Paul, a Roman citizen, in chains.”
(The New Testament, Acts 22:28)