The fundamental error in all cases is to conceptualize insider trading as buying from someone who would have bought/sold anyway. This is precisely failing to think at the margin. It is as erroneous as saying "eating meat is ok because those cows would have been killed anyway". Put more technically, when you buy a share, you do so by shifting up the demand curve a tiny bit, with your demand, which in turn shifts the price slightly up and causes a seller to sell, who would not otherwise have. Market microstructure, together with the fact that supply/demand curves really form a single curve, can obscure this fundamental economic fact.
Given this, insider trading does cause harm to some people. And how could it not? If a person can make money from insider trading, then, to first order, someone else must lose money. There are some externalities from information revelation, but only a tiny fraction of these benefits go to the marginal buyer/seller who lost out because of insider trading.
How does this compare to public releases of information? Well unlike insider trading, public information can shift prices without any transactions occurring (or in practice, very few). This is because while insider trading only moves the price by the mechanism of moving the supply/demand curve, while public information is revealed to all traders at once.
So while insider trading does reveal information (which is a good thing) it does so in a way that reduces liquidity, because people don't want to be on the wrong side of insider trading.
I'll admit that the above narrative isn't watertight. I think it's the best analysis that can be done verbally. The only models that allow a meaningful discussion of welfare in the context of financial markets are so called noise-trader models, which explicitly model the (irrational) reasons why most people trade. The whole field is vastly complicated by the fact that theory predicts almost no trade in stocks if people were completely rational.
[0] http://www.marketwatch.com/story/why-insider-trading-should-...