Labelled as spoiler because of Manga stuff.

Here's the theory: The real Hisoka is dead. What we're seeing is a far more monstrous being.

When Hisoka made his Nen vow to restart his heart, and subsequently died, he's not alive. That vow took A WHILE to trigger, because it was clear he fully became a corpse. Machi would have noticed a heartbeat within the time of using her threads.

So what's going on with Hisoka is his dying will was to reclaim the power he enjoyed thoroughly, his vow was to restart his heart instead of sustain his life, so we're seeing essentially a Nen-produced Revanant. — For those unaware, a Revanant is if a ghost with unfinished business but controlling an undead body.

"But what of the Prince with the Revival Cat?" I expect you to reasonable inquire, WELL there is an answer! — Her Revival Cat takes the life force out of all the cells and essence of whoever kills her, and then immediately revives her in less than 60 seconds. Idk, but I'd argue Hisoka's revival was more than that, probably over 10 minutes.

So now we have Hisoka, who was downed long enough to be declared dead, be transfered elsewhere, and then come back well after brain death? That's a fucking undead monster now.

And we already know souls exist, a la Chimera Ants. We know undead entities can exist, because of Neferpitou specifically with Kite's body. We know Nen can violate the laws of Dead between 2 different arcs. Hisoka his not the guy we know and love to hate he was before or during his fight with Chrollo. He's dead. This current Hisoka is a Nen produced undead monster bent on killing others and the Nen itself knows what it can do in his body.

So that begs the question: How does one kill that which is already dead? I supposed we'll need to hope Shizuku's vaccuum can absorb his corpse, even if the Nen remains