I’m not sure you’re correct. Shalit costed a thousand prisoners with blood on their hands. I don’t think there’s much more value to Hamas with 240 than with 24. How does the scale tip Israel’s way if they start killing hostages? Au contrare. Killing x amount of hostages daily that a deal isn’t reached would probably up the ante for Israel tremendously.
If they kill the hostages, Israel can destroy them without worrying about the hostages. Obviously losing the hostages is a loss, but the rest of the war can be more intense without hostages around. If they keep just a few hostages alive as human shields and bargaining chips, it's easier for Israel to find and track their locations and work on an operation to extract them.
The value is definitely not the same for 24 vs 240, because they can get concessions for hostages 25-240. They may not get the same as the would for the last remaining hostage, but they definitely have a benefit from it and Israel needs to give them enough that it's worthwhile for them to release the hostages instead of killing them.