To be fair, Pre-TS Law also said that he wanted the One Piece. Fast forward to Post-TS and then he is revealed to only desire Doffy's demise to the point of self destructive obsession.
The reason folk have two different perceptions of Law is because there are essentially two different Laws.
There is the shit stirring, laid back, competitively petty Law who is a (supposed) rival to Luffy for the One Piece and then there is the super serious, strategist Law who only cares for revenge against Doffy.
The former is what we got Pre-TS and after Dressrosa. The latter is what we got during Punk Hazard and Dressrosa.
The reason for this occurrence is that Oda has admitted himself that the Supernovas were a last minute addition that were created before the very chapter that they appeared in.
The Supernovas were not planned and Law was never meant to be as important as he became which is why his character isn't nailed down properly because his character was never even planned to begin with.
Another example of this is Bege. Pre-TS Bege ruled his crew through fear and would kill subordinates on a whim if they so much as gave advice against an objectively questionable order. Post-TS Bege is a big softy that cares for his crew as a whole and is a far cry from the cruel subordinate murderer he was initially portrayed as.