From an operational-morality stand point it would make sense "bad person gets punished". The issue here is that once and again Togashi writes the characters and events, in a way that consistently challenges operational-morality (with the CA arc being the best example overall).

If you want an answer to why he did it he says so himself, the battles done, the enemy has been defeated. He will be delivered to a prison and probably die doing time. There's nothing to be gain from having him hurt in the floor (maybe not mortally hurt, but hurt nonetheless). Is the same basic concept of, why would you deny emergency treatment to a suspect that got shot during an arrest? Even if they're not mortally wounded, what is there to gain in further increasing their suffering? Rather, punishment for the sake of punishment is not precisely fair, righteous or very ethic.

Honestly, I can't get behind your logic of "the one scene that almost ruins the whole thing for me". If it did ruin the whole thing I would consider you to be a very petty person. Even If I don't agree with the decision (although in this case I do agree with it) a story that actively challenges my moral expectations is part of why I fell in love with Hunter X Hunter (is not like it didn't do it several times before). So I just can’t get behind the idea of a character’s action defying my expectations as being something bad, even if I completely disagree or highly criticize their decision.

Also, Biscuit was willing to kill Binolt as both a threat (to keep him in line) as well as a form of insurance (until he was willing to turn himself in). This is the same type of post-operational morality expressed during the situation with Genthru. Violence is not a means in itself, if the end has been met, any further violence is not only unnecessary but also unjustified.

And the value of the card is irrelevant here. They had already decided to 1) not kill any of them and 2) heal everyone after the battle. There was no grievance from Biscuit or Killua cause their objective was to stop them and basically have then pay (lawfully) for their actions. If anything, I think it speaks volumes not about compassion but about honor in battle, something Gorein just didn’t get (which I actually think fits perfectly as he presents the operational morality stand point and the one most people would consider just, so it helps contrasts both sides very well).