I agree with you about the abuse aspect. That's one thing this could change. I would for example change the voting system to not allow a negative score. It doesn't happen often, but it still doesn't feel good that people disagree and not only do they not upvote, but they downvote until you get negative score.
In my opinion if you disagree don't upvote, or at least downvote until 0. So people know the overall sentiment is negative, but there's not need to show how negative it is. If it's so bad to be offesive or innacurate, then report it. But downvoting until it's negative feels really overkill and feels like at the moment everyone not only disagrees, but hates you.
The constructive replies you talk about depends on the subreddit. There are subreddits that are literally lifesavers. For example subreddits to overcome emotional abuse and abusive relationships, you'll find support there that is simply amazing and not abusive at all. Or subreddits about certain niches that people are passionate about.
Unfortunately toxic people are sadistic and get a kick out of abusing people, so it's bound to happen. Best way is to report it, so the moderators either warn the person or ban them.