For the source text click/tap here: Bava Kamma 67
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Rebbi Akiva in a Beraisa says that the reason why a thief who slaughtered (Tavach) or sold (Machar) the animal he stole must pay back Arba'ah v'Chamishah is "Mipnei she'Nishtaresh ba'Chet" -- "because he became rooted in sin." What exactly does this mean?
(a) RASHI here writes that it means that the thief "made roots, meaning that he strengthened his sin by acquiring it and benefiting from his deeds." Rashi means that until the slaughter or sell of the animal, the thief did not acquire the animal entirely, but rather it was still considered in the possession of its original owner. (The thief has only a "Kinyan Gezeilah" which removes the animal from the "domain" (Reshus) of the owner, but not from the "ownership" (Mamon) of the owner.)
When the thief slaughters or sells the animal, he acquires it entirely. His deed removes the animal entirely from the ownership of the owner. "He strengthened his sin" refers to the sin he did initially when he stole the animal.
We explore the differences between Arba'ah v’Chamishah 4x and 5x and possible reasons.