I can imagine your reasoning, and am obviously much less familiar than you with the site's various common scenarios and edge cases; however, I think should at least be obvious that the number changed meaning, maybe using some CSS change that is more visually obvious than the appearance and disappearance of the minus sign.

I agree that it’s not perfectly clear but i thought it was better than summing them. I could be wrong

k00b

I'm seeing negative numbers on comments that only received downzaps; zapping this causes the number to be replaced with the positive zap amount, rather than summed. This seems a little unintuitive, as the information of the downzaps is now lost from my perspective, although still relevant in the calculations...