I always wondered this as well and felt it was out of the pattern and for him to kill his own biological child.
My theory on Rea is that Pepper took off after seeing his violent tendencies sometime shortly after she was born. Just guessing by the isotope traces, they likely ended up in Minnesota or somewhere near there. I think TPR likely tracked down Pepper or maybe she was sucked back into contact with him at some point. I believe that he took Marlyse and children with him to get her on the way out to New Hampshire. I believe without Marlyse’s knowledge, he likely killed her and took Rea with her. He may have set up camp near by for her and then children or stayed with someone then went to find Pepper and Rea. Then likely dispose of her body, like in a concealed container, in a different location on the way to New Hampshire (again unknowingly to Marlyse). I believe that is why we see his first alias appear after that. It’s not like he had much of a criminal record to run from at that point that He was running from . I think he likely convinced Marlyse to believe that Rea’s mother was not a fit mother and they had to hide from her. He probably played the story that it was in the child’s best interest to bring her with them.
This may be why he was able to convince Marlyse to possibly changing her name as well. I do not believe that Pepper’s remains will be found near anywhere that she had lived previously. Again MOO based on study different facts from the case over the years.
Marlyse may have been on to his lies or may have caught him abusing one of the girls which led to him physically lashing out. Or maybe he just lashed out it a drunken rage. I am not totally sure on that one. But during that incident, all 4 ended up as victims and disposed of in the barrels.
Somewhere I remember hearing or reading that his old boss at the mill (or maybe an employee) would hear him sleeping in a locked room there and he would have violent fits in his slee
These are my thoughts, too.
I think Rea was a witness to the murders and it's likely why her father killed her. I think Pepper could have been living under an assumed name, hence why she hasn't appeared on any public records after 1976.
While there are some really good points, murdered because of being witness doesn’t seem plausible enough. She was killed in age 2-4. So her memory is not fully developed by then and also she would not be a reliable witness for that. Someone like R, or the killer, who had a good understanding on how the system worked and who was cunning, would not be concerned about a 2-4 year old child being a witness, I think.