This Silver-Moon Demon Wolf was Jiang Yi's first spirit beast, and it might not have high spirit wisdom—not even comparable to a little puppy that knew how to gain the liking of the owner. However, this demon wolf had been with Jiang Yi for a period of time, which Jiang Yi had developed some feelings for.
Right now, the situation was too dangerous; and if Jiang Yi didn't sacrifice the demon wolf, he wouldn't have any other ways to gather all the flying ants together for a concentrated attack. If the flying ants couldn't be eliminated, Jiang Yi had to rely on the Devil Sky Silk to survive and would eventually be blasted to death by the people from Water Moon Observatory.
If he was to die, the Silver Moon Demon Wolf would die, too. Hence, he had no choice but to sacrifice the demon wolf.