A con, or confidence game, is a swindle — when you take advantage of someone's trust. If you con someone out of their life savings, you might wind up a con — as in convict.
The word con has many meanings, none of them good. An argument has pros and cons, and the cons are always the downside. An ex-con is someone who spent time in jail as a convict, and a con artist is someone who scams people out of their money.