As far as Sodom and Gomorrah, God did not make a mistake, He already knew exactly how many righteous people there were (and weren't, to be more precise) in that land. The exercise of discussion was for Abraham's benefit, not for God. God was also showing His willingness to spare the town, if certain conditions existed (which He knew did not). The people who lived in those conditions were the ones with sin, not God who caused them to be that way. They made their own choices to sin exceedingly abundantly.
Concerning Paul, he was stating that we all sin. As far as he moreso than others, read Phillipians 3, where he also said he followed the law moreso than anyone (he followed the law, of anyone, most completely, a Pharisee of Pharisees). But even though he followed the law, he was convicted by God when Christ appeared to Him after the resurrection and asked Paul why is he persecuting Him (because he persecuted the Christians). I believe that may have been what Paul was referring to: that he, Paul, by persecuting Christians was actually persecuting Christ--a greater sin than what we do to ourselves:
Phillipians 3: (3) For it is we who are the circumcision, we who worship by the Spirit of God, who glory in Christ Jesus, and who put no confidence in the flesh-- (4) though I myself have reasons for such confidence. If anyone else thinks he has reasons to put confidence in the flesh, I have more: (5) circumcised on the eighth day, of the people of Israel, of the tribe of Benjamin, a Hebrew of Hebrews; in regard to the law, a Pharisee; (6) as for zeal, persecuting the church; as for legalistic righteousness, faultless. (7) But whatever was to my profit I now consider loss for the sake of Christ. (8) What is more, I consider everything a loss compared to the surpassing greatness of knowing Christ Jesus my Lord, for whose sake I have lost all things. I consider them rubbish, that I may gain Christ (9) and be found in him, not having a righteousness of my own that comes from the law, but that which is through faith in Christ--the righteousness that comes from God and is by faith.