Why was Cain from the bible afraid that someone would kill him?

Why was Cain from the bible afraid that someone would kill him? Besides Adam, Eve, Cain and Abel, who else was walking the earth that put fear in the heart of Cain?

From Genesis:

"You will be a restless wanderer on the earth" (Genesis 4:11-12). In response, Cain lamented, "My punishment is more than I can bear. Today you are driving me from the land, and I will be hidden from your presence; I will be a restless wanderer on the earth, and whoever finds me will kill me" (Genesis 4:13-14). God responded, "Not so; if anyone kills Cain, he will suffer vengeance seven times over." Then the Lord put a mark on Cain so that no one who found him would kill him" (Genesis 4:15-16).

Apparently, there were other beings walking the earth and Cain was afraid they would hurt him so God had to put a mark on him. Who were these beings? Where did they live? Some people point to Genesis 5:4 to state that Adam fathered Seth. Ok? Soooo.....Cain was terrified that his little brother Seth was going to kill him?

Why all this trouble and confusion? Why not create beings that weren't murderers in the first place? Free will you say? Ok....so is God all knowing? If the answer is yes, then he knew, along with the creation of free will, that there would be certain beings that would be murderers. and he knew who those beings would be. I mean he is the ALL knowing right? If God knew all this up front, he's a pretty twisted creator. Or did God not know that Cain would be a murderer and God had to wait to find out how this new fangled free will thing panned out? If so, then God is not all knowing and not the source of all knowledge so there is something greater.

We could continue down this rabbit hole but a big part of the confusion begins with language translations, mistranslations, and transliterations, all of which have led to much of the religious confusion on the planet today.