Answer

This is not a temporary death but an eternal death. Ordinary death too is only a temporary death until the resurrection of the dead occurs; it just takes longer there.
Not everything that happens for a limited time is considered "temporary." For example, someone who buys land in the 48th year, right before the Jubilee, is no different from someone who buys it in the first year. Moreover, in both cases this is ownership of the property itself, not merely rights to its produce, even though it is for a limited time. The reason is that the Jubilee is an annulment by the King, meaning it is an expropriation and not a predefined time limit. For example, if I sell you land forever and after a year the king comes and expropriates it, it is clear that this is not time-limited ownership. Your ownership is permanent and ownership of the property itself. The reason is that time-limited ownership is ownership that was defined from the outset for a fixed period. Here, the ownership was made permanent, except that the king came and confiscated it. The end point was not built into the transaction itself.
And so too with temporary life. If it is known that the illness inside you is what will kill you within a short time, that is temporary life. But if it is only known that you will die, yet the cause is not present within you, that is full life. This is similar to the distinction made by Tosafot in Bava Kamma between throwing a vessel from a rooftop, where it is considered a broken vessel, and shooting an arrow at the vessel, where it is not considered broken, because the cause of the breaking is not yet inside it.
True, in the case of the Jubilee this is known in advance, and still the transaction is a permanent sale and it is the Torah that annuls it. Therefore there too it is ownership of the property itself. And likewise in the case of shooting an arrow, where it is known that the vessel will break within a few seconds, still, since the breaking is not yet inside it, it is not considered a broken vessel.
So too in our case: when you kill him, this is not a partial or diminished killing. It is complete killing, and it carries the prohibition of murder. The fact that afterward the Holy One, blessed be He, will "annul" the death—that is, change the situation—is not built into your act itself. You killed completely, forever, and only afterward the Holy One, blessed be He, comes and revives the dead. One could discuss a case in which you yourself have the power to revive the dead, and you kill a person and revive him immediately, but even there it seems to me that this is full-fledged murder.