Er, I think jealousy is more than enough reason for him. I think you need to rewatch the earlier parts of the episode that go into detail how much of an ass Furuichi is. He obviously disliked Akiyuki since before, which is shown by his comment about how Akiyuki was always "cool" when the three were together (which has nothing to do with him being a humanform).

It may be true that Furuichi hates humanforms unconditionally, but do you really think his decision to point a gun at Akiyuki (who is supposed to be his close friend) and telling Haru to shut up when she defends him does not involve jealousy and frustration from getting ignored by the girl he likes?