Different relationships with the MC's mothers would help, but I think one of the most important things would be differences in setting. If this is true:
The first one, the monk MC never knew his real mother, but eventually finds out. She would be an evil, demonic worshipping witch.
The second MC, who was left for dead by the dad, his mother was a demon and he knew her as a very young child, witnessing her death at the hands of the father and the father's attempt to kill him as well. She would be more neutral, maybe somewhat inheretly evil, if her duty was to eventually plan to sacrifice her son, once he hit puberty.
The third one, the MC who was adopted by two loving parents. The adoptive mother was full of kindness and gentleness, who he cherished. Having grown up with her, then the family murdered, including her, all taken from him at the age of fifteen. His real mother was an angel, he discovers later on.
How would a story progress if the main character had a quite happy and normal childhood and the parent's influence was not a factor
You mean create another MC, where it had nothing to do with the parents at all?
As a reader picking up your second book, I would probably think, "Another wounded son story that ends in a Mortal Kombat-style climax?"
It's a problem. I wish I could fix it. Maybe if I merge the first MC and second? But how can that work? After the dad kills the demon mom and child MC, the child MC survives, then found by an old master that takes him to a monastery to be raised and trained, then when the monastery is raided years later, the MC is the lone survivor who also accidentally killed the master and a few of his training members, then enters the death battle tournament for revenge? Is it too much?
the others have said, don't be afraid to push yourself beyond your initial plots. Oftentimes our first ideas aren't our best. It's the third or fourth ones that are gold.
Thank you, but I don't know if it's working