It's probably that, but suppose it's not...
Young, inexperienced boy, charging boar with wooden sword. Father steps between the boy and the boar and gets stabbed by the sword. If that's the case, then it's death by Darwin.
My thought is it's neither this or the quoted thing tbh, because the hit on page 17 seems very intentionally ambiguous to me, with us neither seeing the stick or the boar hitting the dad. It's not that the kid struck the monster first, or that the father gets stabbed by the sword, because we don't see the hero actually hit anyone here. I think what happened was:
-Kid jumps out to try to fight
-Father sees kid, tries to shield kid (before the kid can manage anything)
-Father gets struck by the boar on page 17
-Kid freaks out about his dad getting hurt and blacks out
-Kid, while blacked out, and during panels which we have not been shown yet, awakens to hero power and kills the boar
-Townspeople find them and bring them back to the village
Which is why:
At most it was a missed hit. If he can't tell the difference between a human and a giant boar, he's got bigger problems.
Because despite being "unconscious", he was still conscious enough to feel some kind of visceral sensation, and due to his current freak-out mental state, he assumed it must've been his dad instead of the boar.
But considering the boar was right there, and seemingly unharmed by the time his dad got hit + the hero "lost consciousness", it doesn't make much sense to me to think that they miraculously got rescued by someone else in time for the hero to not have also been attacked, so the boar had to have been defeated by either his dad (unlikely) or the kid (after awakening to hero power). The hero also mentions that he left after hearing his dad was dead, I wouldn't be surprised if this was also a case of "the anime character ran away due to a misunderstanding before anyone could clarify things to him", which they'll finally get a chance to now.
Just seems like a classic case of a "weak hero kid awakens to his powers in an emotional event" story beat.