Halkenburg is walking a tight schedule with an undefined victory condition, at least as far as I understand.
His intention is to kill Benjamin, I can't see it being anything else... well, that isn't quite right, but the other options seem out of place(?).
I won't try to decipher his intentions and train of thought too much, Togashi gives hints and clues but never the full picture until necessary.
But the essential part is... what happens after his ability effect runs out, and if I understood it right... he is betting on his Nen beast having some sort of fail-safe effect supported on the "restriction and reward" condition that he is the target of soul transfer and the beast target of protection, it will somehow "resurrect" him... a little farfetched, but not impossible, with the right conditions meet, and the fact these beast are high level nen constructs, I can see it happening.
But going back to Tserriednich... Theta, you are too on a tight schedule, pray that his talent isn't so ridiculous as for him to break that barrier quickly.
But make no mistake... this crazy fucker can break it, so make a plan quick, because when he achieves that... he will start experimenting, and whatever he did in the past will look like child's play compared to what he will be able to do.