There is a quote from the History of Middle-Earth (Morgoth's Ring) - supplied in dlanod's answer - that states:
There is no doubt that long afterwards, in the Third Age, Saruman rediscovered this, or learned of it in lore, and in his lust for mastery committed this, his wickedest deed: the interbreeding of Orcs and Men, producing both Men-orcs large and cunning, and Orc-men treacherous and vile.
OK, I get it that creation of Orcs as an act was a Bad Juju by Morgoth (or Sauron?) originally. But:
Why is merely breeding men and orcs so bad now that Orcs exist? Somehow, breeding Elves and men is a Highly Commendabe Thing (Aragorn and Elrond are both products of this).
Even assuming that the breeding was not consensual on human part and this was the reason for marking the deed bad, this surely was not - by far - the wickedest thing Saruman ever did.