Who was the man that Eliyohu killed a reicarnation of?
The gemora in Berachos 6b writes that there was once a man who prayed behind a synagogue and did not turn his face towards the synagogue. Eliyohu passed by and saw him and appeared to him in the guise of an Arabian merchant. He said to him: You are standing before your Master as if there are two powers who govern the world! Eliyohu drew his sword and killed him.
That person was a reincarnation of Ach’av who had believed in his lifetime that there were two powers governing the world, and even after he was reincarnated he still persisted in this belief. Thus Eliyohu said to him: You still persist in rebelling against Hashem! He then drew his sword and killed him because Ach’av had pursued Eliyohu in order to kill him (and consequently Eliyohu was forced to live for some time in a cave), and now Eliyohu killed him.