You are right 100%. As all other religions, Paul too finds it difficult to explain sin and its expiation. In the process, people come up with some explanation which would not stand under scrutiny!
Correct explanations should meet the following criterion:
1) Man does acts of virtue or vice because of his own volition and choice, hence he is 100% responsible.
2) This would mean there will be variety of personalities—good, bad and those in between with mixed qualities in varying degree.
3) The reward should not be just heaven and hell, but each one should get varying reward according to the degree of merit or demerit one has earned. (Jesus hinted about this when he said: “with the measure you use it will be measured back to you.” (Mathew 7:2; Mark 4:24; Luke 6:38) but he gave no elaboration.