Mermaid Man and Barnacle Boy would fall into the 3rd stage of moral development. Although one would think that they would be at the 4th stage, seeing that they are saving Bikini Bottom from evil and upholding social order, the motivation behind it is what keeps them in the lower stage.
In the episode Mermaid Man and Barnacle Boy, Spongebob and Patrick try to convince their heroes to come out of retirement in case evil was to strike Bikini Bottom again. At first they speak to them about how they need to be around to enforce the law and to just stop evil. This does not make them budge. It is not until they mention how they idolize them and how they would be famous and loved by the people of Bikini Bottom if they were to make their comeback. I believe this is evidence to place them in the 3rd stage of moral development.
In the episode Mermaid Man and Barnacle Boy V, Barnacle Boy is finally so fed up with Mermaid Man calling him "boy" all the time, that he lashes out and joins up with Manray and The Dirty Bubble, becoming a united EVIL. This is an example of Barnacle Boy acting at the 1st level of moral development. He is doing this merely to make himself happy, not wanting to be insultingly called "boy" by a senile old man who wears a starfish bra anymore. Eventually, with help from Spongebob and Patrick, EVIL is defeated and Barnacle Boy returns to his old self again.
