The whole point of Re-educating a person is so that others around him realize the folly of believing in anything else besides the party and it’s ideology. If he himself states that he is wrong and even believes he was wrong then who would follow his example?

After that purpose is served, the Re-educated person becomes a loose end. So might as well kill him thereby sending another message to the people. The simple message being follow us or die. It just reflects the brutality of the party.

Or maybe an over indoctrinated person is liable to go back to his rebellious phase after a certain period of time.

So the actions and beliefs of a re-educated person are only temporary and so the party would just kill them to prevent their real selves from re-emerging.

An Outer Party member like Winston would be used to what were known as "unpersons". They would be taken away, vaporised, and their names struck from every record, article and photograph. Winston himself even partook in this process. It would be a crime to refer to an unperson even for the unperson's family. "There is no Newspeak word for what happened to unpeople, therefore it is thoughtcrime to say an unperson's name or think of unpeople." If Parson's was foolish enough to say, "Hey Smith, where is Sime today, he owes me a razorblade?" Then expect, Winston squirmed in his chair his mouth opening and closing as uselessly as a grounded fish. This fool Parsons could bury them both. "I don't know of whom you refer brother, perhaps you are mistaken." Winston offered. Parsons realising - at last - his stupidity, mumbled an excuse and sat silently gazing into his meal. He said nothing after that. In the bustle of the canteen Winston was unsure if the remark had been picked up by the telescreens or hidden microphones, or an overzealous party member. The above scenario is not a quotation from the book but elaborations on my part. "… the endless purges, arrests, tortures, imprisonments, and vaporizations are not inflicted as punishment for crimes which have actually been committed, but are merely the wiping-out of persons who might perhaps commit a crime at some time in the future." Syme was just such a man this happened too. Unquestionably Syme will be vaporized, Winston thought again. He thought it with a kind of sadness, although well knowing that Syme despised him and slightly disliked him, and was fully capable of denouncing him as a thought-criminal if he saw any reason for doing so. There was something subtly wrong with Syme. There was something that he lacked: discretion, aloofness, a sort of saving stupidity. You could not say that he was unorthodox. He believed in the principles of Ingsoc, he venerated Big Brother, he rejoiced over victories, he hated heretics, not merely with sincerity but with a sort of restless zeal, an up-to-dateness of information, which the ordinary Party member did not approach." Sounds like a good party member, huh? Not so... "Yet a faint air of disreputability always clung to him. He said things that would have been better unsaid, he had read too many books, he frequented the Chestnut Tree Cafe, haunt of painters and musicians. There was no law, not even an unwritten law, against frequenting the Chestnut Tree Cafe, yet the place was somehow ill-omened. The old, discredited leaders of the Party had been used to gather there before they were finally purged. […] Zeal was not enough. Orthodoxy was unconsciousness." When Syme disappears Smith knows fine well he has been vaporised. To even think about him after that realisation is dangerous ground.