The traditional and decent way to slaughter a servant is to allow her to masturbate and chop her during orgasm. Often she would spend the day before being pleasured by others in preparation. This not only eases her mind but makes her taste better. To be slaughtered "cold" instead is a severe punishment, especially while bound, and especially if the body is discarded or otherwise not treated as being as valuable as usual. A servant can regain some honor by submitting regardless.
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It's often at adolescence that kids start rebelling and showing their ugly side, or otherwise being disappointing. But this is a head problem, and now heads can be replaced. Girls(and boys?) are raised by professional institutions as spare parts; Internal organs are kept healthy, and heads are taught to be good and obedient and ready to assume the persona of the one they replace. They are treated as the same person who has undergone some changes, and see themselves accordingly. Possibly they are clones prepared in advance, or their heads or bodies are preserved for future use when the other has been taken. Maybe they don't always bother with a head transplant but just replace the whole girl and eventually scrap the unwanted one. Maybe they switch their heads to keep the carefully-prepared organs alive.
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As tourists gather, girls line up so that a skilled person with a big knife can chop off their heads, and with a just a few more moves extract their intact brains from their skulls and toss/sell them to the crowd. Maybe it's done so well they are still conscious when bitten into(or people are harder to kill for some reason).