"You see, while we're having our nice little chat, someone out there might be building another Harry right now, and that one isn't even bound by an Unbreakable Vow. Prophecy happens, everyone dies. I offer you the end of the world on your terms. Are you really sure you wish to terminate me?"
Yes, that particular plan is highly improbable, and LV can search the globe for Harry-builders in his own time.
The elements of this that are threatening are: if you kill me that might no avert the prophecy; and if you kill me I might come back to haunt you (means unspecified in both cases). The standard answer to the former is that if prophecies can't be averted then this is all a waste anyway, so LV might as well try to kill Harry. The second is harder, but I model Voldemort as rejecting this, although I don't quite know why.
tl;dr avadakedavra
"You see, while we're having our nice little chat, someone out there might be building another Harry right now, and that one isn't even bound by an Unbreakable Vow. Prophecy happens, everyone dies. I offer you the end of the world on your terms. Are you really sure you wish to terminate me?"