1. Sloth - Patrick: Sloth is the sin of laziness, or of unwillingness to act. Obviously, this is Patrick. He lays under a rock all the time and doesn't really do anything. In fact, in the episode "Big Pink Loser", he got an award for doing nothing the longest.
2. Wrath - Squidward: Wrath involves feelings of hatred and anger. Squidward hates his life, usually hates SpongeBob, and is angry most of the time.
3. Greed - Mr. Krabs: Obviously, Mr. Krabs is greedy and desires money. How could Greed not be Krabs? He actually sang about the power of greed in "Selling Out".
4. Envy - Plankton: Plankton is envious of Mr. Krabs, because The Krusty Krab is a success, while The Chum Bucket is a failure. His envy drives him to try to steal the Krabby Patty secret formula.
5. Gluttony - Gary: I actually think this one is pretty funny. Did you ever notice the running gag in SpongeBob, where they say "don't forget to feed Gary" or Sponge says "I gotta go feed Gary"? Gary even ran away when SpongeBob forgot to feed him. Gluttony usually refers to the overindulgence of food, so I'm guessing this one fits him pretty well.
6. Pride - Sandy: Sandy takes a lot of pride in who she is and where she comes from. She takes pride in the fact that she is from Texas, and likes to let everyone know it. She also takes pride in the fact that she is a mammal and a land creature, For example, the episode, "Pressure", where she tried to prove that "land critters" are better than "sea critters".
7. Lust - SpongeBob: Okay, I know what you're thinking. It does seem a little weird and curious at first, but I have given it a lot of thought. Lust in one definition is "excessive love of others". I think this one works best for SpongeBob. He shows his love of others with his overeager need to do good and help people. If anything is true about SpongeBob, it's that he loves everyone around him, even if they don't exactly love him back.
So this is what I came up with. I don't know what anybody else thinks. Oh, by the way, try not to analyze cartoons too much, especially SpongeBob.
I actually think the people on the show try to be inconsistent on purpose. Just to be funny. Or are they trying to tell us something that we weren't supposed to know about the beginning of this show, if its true, then what will happen next...?