He will not hesitate to kill his people no matter how many are they if they double cross him or if they become a snitch of the police. He narrated that there was a time he killed everybody because they are putting his business in danger by conniving with the police.
Frank Costello is a “smart guy” which means he got his education in the streets rather in the university. He is a street smart guy rather than a “book smart” guy. The way he quizzes Billy shows his experience in the street such as when he asked if “William” is trying to tell him something when Bill shows resistance with how he quizzes him. Instead however of being threatened, he liked the guy which also shows that Frank likes strong people to be around him. Frank Costello’s childhood and background is not clear in the script but it can be inferred that he grew up in the streets and did not only survive but also prevailed to become the leader of a syndicate
Frank Costello is already an experienced man. In the line 106 where he said “Tell the truth, I dont need pussy any more,but I still like it” is really a figure of speech that he had been through it all and is no longer motivated by beautiful women and money. It is also an indication that Frank Costello is an accomplished man and had all the women and money that anyone could desire. With regard to money, he already had it early in life when he had the “milk money” of Archie in the third grade.
Having all those experiences, Frank Costello is already careful about the business. He knows what snitches can do to him and the business and is doing everything he could to identify and weed them out. He is already old and do not want to spend retirement in jail. This explains why he took the time to quiz Bill in the bar and how serious he is in dealing with traitors.
The way Frank Costello quizzes Bill showed his acumen in dealing with people as well as in extracting information