A question about the injustice of insider trading was raised over at the Acton Institute. The response given was that it harmed someone in someway. This is fairly typical of modern ethical reasoning, which is consequentialist and personalistic, that is a thing is evil when the consequences are evil and especially when the manifest evil consequences are against a person. Evil actions by definition have evil consequences and harm people, the view of the consequences and what is considered harm are generally extremely limited to only that which is manifestly and indubitably evil. However, we wish to consider the evil of insider trading in itself.
To begin with, justice in exchange involves an equality between the property exchanged, namely the equality of value of what is sold and what is used to buy. The market price is generally the just price and is determined by common estimation of the participants in the market, a common estimation based upon available information. In insider trading, there is an asymmetry in the distribution of information such that someone has information the rest of the market does not. The market price is then not reflective of the just price due to this lack of information. This is seen in that once the information is made public the market price is corrected.
The additional information that the insider trader has gives him knowledge of the just price which differs from the market price. This allows him to trade at the market price while knowing the difference. This difference of value in the exchange is unjust. The injustice of insider trading is a matter of the act itself rather than someone somewhere being harmed.