Al D'Arco claims agents came to his house and warned him that an attempt would be made on his life. But prior to that, he says he already had the card of an FBI agent that he'd ran into on the street. The fact he kept the card implies that he already had an intention to flip.

He says in the book that he knew he and Sal Avellino were going to be hit that night at the hotel. Yet Avellino stayed and was unharmed after D'Arco hightailed it out of there. And remained a large part of the Lucchese hierarchy the years following that supposed hit attempt.

I don't think D'Arco was going to get whacked. I think he was mad that he got bummped down from acting boss and was put on that panel. So he turned informant.