What is the difference between a good and bad SPY?

Not talking specific countries, but a good spy works in the best interests of the country he gets his paychecks from, a bad spy works for whoever is willing to pay him the most for his services. All spies are trained to do lots of things, most are well versed in hand to hand combat have some weapons training and for whatever country they are in have at least a basic knowledge of that country but can imitate someone from a country other than where they are from. When on foreign soil some of the rules we follow go out the window they will lie to anyone to get what they need, take whatever they think they need and can blend in quite well. A spy can be an in and vice versa. Now the FBI informant mentioned in the message above, just means you have a guy who is giving information to the FBI on someone or some group of people or something that they would have an interest in.