Why was he troubled that he was being driven out from the face of God? Even Cain felt a need for God, and need to worship God. He had been engaged in an act of worship when these events had started. Unbelievers and not unaware of their need of God. Cain was right to say that he would be shut out of God’s presence, for God would not receive him when mankind and even the soil itself rejected him.
Well, where do the ‘everyone’ people come from? Plainly, you see there has been time before this event for other children of Adam and Eve to grow up, and for the region to begin to be populated, with few or many.
Why should they want to slay him? Well, because those first people had the law of God in their hearts, and there was an abhorrence for murder, and there would be a natural rejection on their part of any murderer. Cain realised this – murder is abominated. Everybody agrees, it seems, that the worst thing you could do is take a life. So he fears for his own survival, and he fears that he is going to be killed by anyone and everyone. His fear may not be entirely realistic, but it does show that it was in the heart of man to be appalled at murder from the very beginning. So he complains.