Kindly look at these these pages.
The baby was already there under the tree and the dead woman just as Gambino's band was passing.
Gambino commented on how creepy the sight was. After that Shizu got off the carriage and picked the baby up. The soldier asked if the miscarriages Shizu had before were his kids. He wasn't referring to guts.
These clearly show that Guts wasn't Shizu's son at all. The pages galvanize that fact. And the people hanging on the tree weren't their victims nor part of their group. They just came across them.
But then again, there's also nothing to say that he's half human. And you're right, I would prefer to see Guts rip Griffiths throat off using his bare human hand. But before he could get to that point, he would need some magical upgrades, I guess. And yeah, I miss seeing apostles faces in utter shock and disbelief as to how a mere human, one eye is crushed and wearing a prosthetic arm, could slash and annihilate them.
By the way, the tree were guts was found is similar to that tree where forest druids were said to sacrifice people.
A crazy theory, what if Guts parents were Druids who were executed by the church because of believing in nature spirits. And the story about them sacrificing people were just propaganda by the church. The opposite was what was actually happening. The church was executing all druids at the tree. Another thing, the robes of those people hanging on the tree are similar to the druid's. robes.