You can only make a statement like, By the standards of a valid system of ethics, however, you're not. if you presume to be able to determine for others what a valid system of ethics is.

You might think that you can do that, but you'll find that others disagree. For instance suppose that I go into work and paste code written by others at my job to Perlmonks. How many people think that it is now right and fair for Perlmonks to do anything that it likes with that code? Do the people who think that Perlmonks should not compound what I did in that hypothetical example have invalid systems of ethics?

Morality is a far more complex topic than most people realize. And if you're a moral relativist, like I am, it becomes harder still.