Monopoly businesses. Net loss of utility through inefficiency.
However, increasing returns to scale may make a monopoly a more efficient producer than a non-monopoly. In those cases the efficiency loss due to the lack of competition may be more than cancelled out by the efficiency gain from exploiting returns to scale.
The "canceled out" part depends on whether your interested in the utility of stockholders and the reduced resource consumption of the manufacturing process or the utility of the general population which might have to consume less of the product than they'd otherwise be able (because of higher prices) or more generally have less capital left to buy other things they need/want. Monopolies with regulated price structures sometimes work, I guess, though it's complicated.
This is an offshoot of a thread I made earlier, but which wasn't eliciting the sort of responses I'd hoped for.
So let me pose a clearer question with less potential to get people on watchlists.
What legal ways of making a profit are the most anti-altruistic, the most damaging to society, the opposite of effective altruism in result.
I am using utility loosely. The answers need not be given from a utilitarian perspective at all, but instead merely deal with any means of making a profit that seems to you clearly wretched, and such that the world would be better if nobody participated in it.
I'd also like to emphasize that these things should be legal. There are some obviously wretched illegal businesses that would top the list otherwise. If something is legal but only in a particular jurisdiction, then you should only discuss it within the context of the jurisdiction where it is legal.
If it's a grey area, go for it, but extra points for society-harming enterprises definitely legal in both the letter an the spirit of the law.
This is NOT about whether the enterprise in question should be illegal, just whether it causes a net loss of utility (deal with counterfactuals however you see fit).