I'm surprised no one really talks about this.
On the Season 1 DVD Commentary for "Plankton!" Mr. Lawrence starts talking about how some of the main characters represent the 7 Deadly Sins, to which Carolyn Lawrence says "What's my sin?" and Tom Kenny replys "Yours is Karate!" and they're like "Yeah, Karate is the 8th deadly sin,"
Now it seems like the 7 deadly sins thing they could just be joking about, as it's just a cartoon, but somehow the sins seem to sort of work with the characters, with the 7 being Spongebob, Patrick, Squidward, Krabs, Sandy, Plankton, and Gary. And I've figured that Krabs is Greed, Squidward is Pride, Plankton's probably envy.
This could make an interesting discussion.