If he had all the powers he had when introduced then he could have potentially killed Hashi by pretending to be dead and then stabbing him in the back while his guard is down.
This is actually what should have happened to Kakashi were it not for plot no jutsu. Kakashi sneak attacked Kakuzu and stabbed him in the heart. Kakuzu gets the chance to return the favor and just kicks him instead of killing him, plot no jutsu at its worst.
Having said all of that, Kakuzu definitely never killed Hashi, I don't see how he could have done it if he didn't have the powers that he had when introduced, in otherwords, if he didn't have pseudo immortality.
And if you argue that he had those powers when he faced Hashi, then why didn't Kakuzu have mokuton? Because there is no way you can expect anyone to believe that he wouldn't have taken Hashi's heart.