Most guys turn because they cant handle a long stretch inside.
With that said, D’Arco was as front man( street boss) while amuso amd casso lambed it. D’Arco was pocketing a lot of money that was supposed to go up the ladder and casso and amuso knew it despite D’Arco playing the role of loyal soldier., in the end, he took the money snd ran. To him, it was a business decison. If amuso didnt think he was holding back cash from all the rackets he was collecting from, he probably wouldnt of turned, at least back then. I believe he had a plan. Casso and vic were guilty of many things, being stupid wasnt one of them.