It's not so much a code as the options people had were limited. Once the government/law enforcement agencies started witness protection programs and worked out deals, it gave them a way out. Before that there was no way to leave that life. Much like gangs today believe it or not.

For most of its history the mafia was not a singular organization. Different families have different ways of running things, and. The rules are basically whatever the boss wants to enforce. For example up until relatively recently the mafia in Chicago had no formal process of making someone a "made man", something which is standard in all other families.