Hevel's Murder

Q: Why did Kayin tell Hashem that he was afraid that he would be killed because he murdered Hevel, when only Adam, Chavah and Kayin were in the world? Where his parent's going to kill him?
A: There are two possible answers:
1. It was for future generations. Kayin feared that Hevel's descendants would kill his descendants somewhere down the line.
2. The Sforno mentions in two places (Bereshit 1:26, 2:7) that there were other human beings who they were not created in the image of G-d in the world. Maran Ha-Rav Kook (Igrot Re'eiyah vol. 1, p. 105) also writes in the name of the Zohar (Vayikra, p. 10) that there were various other humans not created in the image of G-d which are not mentioned in the Torah (Maran Ha-Rav Kook also mentions this in Shemoneh Kevatzim vol. 1, p. 584). Kayin thus feared being killed by these creatures.