Israel claims Hamas killed the 6 Zionist prisoners shortly before the IDF reached them.
Even if true, does Israel expect Hamas to keep Zionist prisoners alive while the IDF destroys every sign of Palestinian life in Gaza?
This reduces Hamas' leverage but also increases pressure on Bibi to accept a ceasefire, before all the remaining alive Zionist prisoners in Gaza meet the same fate.
A Hamas policy of killing the captives when any rescue attempt is made would increase pressure on the entity to negotitate for an end to the genocide.
They of course cannot say this openly but the implication of dead POWs whenever any rescue attempt is made would send the message loud and clear to all.