One Million Nights
While I am over simplifying, Roman Morality is based on the concept of “The Rights of the Victor.” If one is strong, wise, and able, one is deserving of power. If one has power, one is to use it responsibly. This is easily shown in the reign of Julius Caesar. With an army at his command, he took Rome; when he came to power, he was beloved by the people. When he was assassinated, Octavian gained his power; partially from being an heir, and partially because of the Mob Rule in Rome.
The greatest sin, then, if one wishes to easily fit Roman Morality into Vampire: The Requiem, is to commit a sin against Humanity without having an over-arching goal. If, by killing someone, a vampire becomes more powerful and respected, the vampire has no reason to believe he has done something wrong. On the same token, if a vampire loses power or respect, he may lose humanity.