That's an over simplication. I don't quite see the problem with shooting condemned people for fun (provided they experience more or less the same amount of suffering).
But I would point out that your example is loaded since I can't seem to see a senario where extermination of the enemies of the United States would be done for fun. The only reason we kill those people is because they are enemies. If they weren't we won't kill them. Whether we enjoy killing them is a different matter.
The crux of my argument is that it doesn't matter if you enjoyed killing ant colonies/terrorists. You still achieved the same result (dead ants/terrorists).
You believe it is morally reprehensible to have enjoyed killing ants/terrorists. I disagree. Does it matter what the executioner feels when he decapitates his subjects?