In antiquity, the concept of morality is somewhat different from what people may consider today. Given that our game world is set upon a historical foundation but also includes the very real presence of deities and their followers, some explanation of the overall theme of morality is perhaps justified.
Because our setting entails numerous cultures and half a dozen separate religions it is necessary to generalize this overall theme, particularly in terms of how it informs the actions and reactions facilitated by DMs on The Rhine. Since everything we do is centered upon striving to maintain a historical basis where possible, there are certain compromises we can afford to make and others we cannot afford to make. When it comes to morality, consistency is king.
Perhaps the best place to look in terms of what the ancient world considered to be among the most horrible sins is found in the description of Tartarus, which the Romans called “The Fortress.”
This lowest layer of Dis or Hell is considered a space reserved for the highest and most serious kinds of sin. Performing actions of these sorts on The Rhine is certain to more than aggravate NPCs and even the gods themselves.
The evilest of sinners included those who showed sin or openly rebelled against the gods. Your PC cannot be like Odysseus and expect to live long with an enemy like Poseidon!
Another example of great sin is being disgraceful among mankind. This is as important to Romans as it is to their Barbarian counterparts. Honor and living up to your word are perhaps even more important for the so-called uncivilized person. If a Barbarian promises to lend you their horse, they will do so even if following through leads to their imminent demise. Keeping one’s word was considered by all to be even more important than keeping your life.
Those who hated their brethren were also considered particularly vile. As were those who were considered to use their parents or cheat their dependents. Even “making no use of one’s riches” is considered a grievous sin, again, all worthy of being cast into the lowest pit of Tartarus.
It should be no surprise that those who committed incest were among this group of the ultimately damned. And also among them are those who disturb a marriage union. On this same level of disdain is just anyone who can be accused of general rebelliousness!
The final segment of those consigned to an ultimate punishment in Tartarus are betrayers of their country. Oathbreaking, desertion, and betrayal of any kind toward one’s country is once again a reason to be thrown into the darkest pit.
While there are always narrative exceptions and the inflections of individual cultures to consider, these points above remain a constant when delving into research and will thereby be a reliable set of parameters to how the setting responds to characters. If you wish to play a character that defies the gods, by all means, do so. But, please be advised that in order to maintain the level of consistency and basis in historical fact that we strive for, things will not go very well for such a character, or if they do, never for very long.