He would have to be one already to do what he did
No. He's very angry after his first misdemeanour with the counterfeit money; too angry to try and get his job back even though his wife suggests he try. In his anger he makes a bad decision to get money in an easy way. Combined with this is his urge to provide for his family, which is a good urge, but it's tainted by his idea that money is the provider. He does not think about the consequences for his family if he is caught, as we see happens. In prison he falls into despair after learning of his daughter's early death and his wife leaving him. He tries to kill himself proving that he is not afraid of dying because he does not love life. One of his cell mates says earlier that fear of death is from love of life. Then one of the architects of his situation is imprisoned with him and Yvon's anger is lit by the flame of revenge. I imagine this continues to burn when he leaves prison. Not having had his pound of flesh he kills others, all in the name of money. He is caught in a madness by this point. He is corrupted by money in the end.
A bird sings and the mountain's silence deepens.
This is a good description as I understood it as well. The story didn't ring 100% true for me, but I can understand it and find it plausible.