
It's instinctual fear. They don't know why they were terrified of their own son either, but their instinct was screaming in their heads that he was extremely dangerous. Instinct isn't something you can just get rid of with logic and reason so even if the parents try to tell themselves its their child and should be raised with love, their instincts will always blare in their heads "DANGER! DANGER! MUST RUN AWAY OR YOU WILL BE KILLED!!!" MC was an exception because of his skill as a caregiver that helped neutralize that fear. Their parents unfortunately didn't not have it.

I thought I remember reading it in the novel that explained that skills are sometimes dormant until awakened like how S class hunters before awakening tends to be intimidating for other people like how the parents were terrified of the younger brother (sorry, I can never remember their names) before the dungeon broke out. A good clue is when MC can see other people's skills that have not awakened yet, they were showed as still dormant until it meets the requirement to awakened. MC's fear resistance to strong presence like S class hunters was due to MC's dormant skill that tho not awakened, it still showed in the form of being unafraid of his younger brother unlike his parents.
FORGIVE THEIR PARENTS! HOW CAN THEY TREAT HYEON LIKE THAT!!