Like Rover said, best case would be to ask them
"//Permission to cut off your finger?" etc
The "//" means you are talking out of character and you must always ask permission in this way before you permanently scar another character. Whether that be dismemberment, execution, breaking bones, or cutting deep enough to leave a scar, anything under that umbrella.
Now if you gain execution rights over that hostage, you can kill them and theoretically eat them, though it can be seen as bad RP, given whoever you executed may or may not PK or permanently kill their character. That gets into a lot of theoretical so best to talk to the person OOC in Discord maybe after the execution to ensure they're going to PK.
If the story line you're going for keeps to lone wolf and you don't wanna deal with OOC and Discord, I personally would not name drop whoever you killed. That again is if you have the right to execute someone, and do so. If you eat them, don't say you ate "Character name", just say you ate a random survivor.
This all may be a lot to consider so never any shame in asking our Mentor program linked here for some guidance if you're struggling still with the rules. Feel free to stop by the Discord anytime for discussions and so forth or if you want, we can assign you your very own mentor to help you out along the way. Better to ask for help then end up in a report!