Do you think Rainsford committed murder or self-defense in the end?

Concerning Rainsford's killing of General Zaroff.
Rainsford had already escaped from the island and, although there was really nowhere
else for him to go but to return again, he chose to confront Zaroff in his bedroom. Upon
seeing Rainsford, Zaroff conceded that Rainsford had "won" the game, and the Cossack
accepted defeat. Although he had already shown that he was a cold-blooded killer, he
also displayed a sense of honor. With his bodyguard, Ivan, dead, he had no other
protection. Rainsford could have chosen to lock Zaroff up where the general kept his
other human prisoners; instead, he chose to continue the hunt and, apparently, killed
Zaroff. There was no evidence in the story that Zaroff was armed and was able to defend
himself; if so, it might be considered self-defense. But assuming that Rainsford killed
Zaroff outright, or even if he hunted him down and killed him, in either case it would
be out-and-out murder--not self-defense.