I think in the situations you describe it's much more effective to have some kind of authority in place to discipline people for this kind of behaviour rather than relying on abuse. Especially since it's usually done by trolls for whom the emotional response is the objective. I think a brief "This is in breach of rule X, you have been banned for Y days" combined with locking/deleting the relevant comment, is far more effective at discouraging negative behaviour than angry tirades.
It depends. I generally try to be reasonably civil online, but I also think there are valid reasons to be less so. A good example is when people are giving advice to newcomers that has a good chance of putting them in imminent danger. E.g. someone goes to an online mushroom identification forum and posts a picture of some random mushroom they found and asks if it's edible, and someone else replies with something along the lines of, "Don't worry, anything you find in your back yard is edible," especially when the mushroom in question is a destroying angel or something. You see similar things happening all the time, with medical advice or otherwise.
Another less extreme example is when people know they're wrong, but they keep repeating whatever they're wrong about because they're being intellectually dishonest to promote whatever agenda they have. A good example of this is my comment the other day on the 'class of 2013: you've been scammed' story, where I tell the guy and his editors he's wrong but he still doesn't correct the article:
https://news.ycombinator.com/item?id=5728788
In these cases a lot of times I find that I correct someone once, but then they actually try to use my civility against me in one way or another, so sooner or later you kind of have to drop the hammer.
If someone is wrong about something because they're just unaware of some fact then by all means be civil. But sometimes there can be underlying issues that are fairly toxic and in order to preserve the community you need to find some sort of way to quash them, whether that's through a technological solution or through some form of social condemnation. In the real world when someone commits a crime like rape we generally don't just gently tell them it was wrong, because we accept that civility is a tool that allows reasonable people to have productive conversations, but at some point people forfeit their right to civility if they're being blatantly unreasonable. The same should apply online.