Most of them rat because they don't want to do the time after doing the crime. Those are the ones that rat after they are arrested and facing charges. I would include Fish in that category. They always come up with the excuse that they were abandoned or robbed or insulted.

Others rat because the family turned on them or they feel the family turned on them. That would be guys like Al D'Arco, Pete Chiodo, and Ken Eto. Chiodo was facing charges, but he didn't rat until after they shot him 14 times. D'Arco ratted because he thought they were going to kill him in the hotel room. Whether that was a setup or not, I think Casso would have eventually ordered his murder. Eto was facing charges but didn't rat until he took three bullets to the head.