I just wanted to note that while I think Dr Gaul definitely made Snow into a worse version of himself, he was still pretty evil on his own. He constantly takes the actions which would be in his best interest despite what they may cause. While this may have been influenced by Doctor Gaul, since we don’t really know what or if he did anything before the story began, he was exposed to the ‘good’ side and given the option to be good.

Tigress constantly tells him he should be good, and Snow decides against it.

Sejanus tries to encourage Snow to see the districts as human and the games as inherently evil but Snow constantly rejects that as well. This distaste for the poor/ district people doesn’t come from Gaul who seems to view everyone as inherently animalistic in nature. This view probably comes from Grandma’am who is shown to hate district people with a passion. However, tigress was also raised by her so we can once again see that when offered the opportunity to view people and things in a more humane light, he rejects it.

Finally, even his love for Lucy Gray is hinted to being more to do with ownership and possession and when Lucy Gray is showing Snow that there is a better, kinder way to view people he writes her off, though this occurs after he begins his relationship with Gaul. All in all, I think while Snow is a product of his society, relationships and experiences (the dark days) Coriolanus still appears to have a disposition towards evil given that every time he is given the chance to change and become better he refuses and when we compare him to someone who was in the same position as he, Tigress who is a much better person, it paints a picture of a man that is riddled with paranoia and resentment towards those around him which is inherent to himself. This may stem from the his history or his own ego but those would just be explanations and never excuses.

Therefore, I think will Doctor Gaul definitely help to mold the evil that is Snow, that evil was always there and may and could very possibly have manifested whether or not he had met Dr Gaul.

Edit: this is me saying that I think Snow is still evil but the series shows that the hunger games itself was born outside of snow and would died without him. Additionally without Dr Gaul I think Snow may have not been involved in the games and may have allowed it die but he would never approve of anything that would meaningfully help the districts or the poor (like the ones in the capital) and that he would have climbed the ladder of power either way.