I doubt that really happens, other than for that very brief moment and tiny minority of users who are right around the threshold. The karma losses are capped, it's hard to lose meaningful amounts of karma without actively trying to.
Sorry, I reread my post. What I meant was that if I downvote someone, the mechanism could deduct from my karma (or a fraction of my karma) as well as theirs.
So we're both at 1000 karma, I downvote you. You go to 999, I go to 999.5. Repeat. If I'm only downvoting, I'm forfeiting my karma to that moderation action. Optionally the same for flagging and other things.
This becomes a constraint because a user who just got to 500 karma and downvotes would lose the privilege immediately. A user at 10k karma can downvote pretty much everything. You could also have it cost more to downvote more in a period of time. Like 0 karma to downvote once an hour. 0.5 karma for two or three times an hour. 1 karma for four times an hour. Some scale (linear or faster).
This would prevent or mitigate people going through and downvoting all of a particular commenters posts in a thread just because it's controversial or they wrote one controversial thing. It creates a cost for moderation.
Oh I see what you mean. You can see a system like that in action on Stack Overflow. I don't think it really does very much at all there either, under what seem like much more favourable conditions: harder to accumulate karma, lots and lots of permission thresholds, a community involved in far more moderation than here and piles of caps on all sorts of user actions.
On HN, this would also favour commenting participation over curating participation. It's not obvious this is a win over the way things work now.
I doubt that really happens, other than for that very brief moment and tiny minority of users who are right around the threshold. The karma losses are capped, it's hard to lose meaningful amounts of karma without actively trying to.