tolkiens legendarium - Why is breeding men and orcs considered evil?

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:

1. 
   

   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).

   
   


2. 
   

   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.

   
   

Answer

The original creation of Orcs, assuming that we accept the Silmarillion account, is described as:

This it may be was the vilest deed of Melkor, and the most hateful to Ilúvatar.

This is in very similar terms to the description of Saruman's works: "his wickedest deed".

Now, even if we don't accept an Elvish origin for Orcs, the description (as a description of a wicked deed) remains valid. What Melkor did, and what Saruman did, is simply this: interfering with the nature of the Children of Ilúvatar.

Here's how the Ainulindale describes the Children:

For the Children of Ilúvatar were conceived by him alone; and they came with the third theme, and were not in the theme which Ilúvatar propounded at the beginning, and none of the Ainur had part in their making.

And here's what Of the Beginning of Days has to say:

For Elves and Men are the Children of Ilúvatar; and since they understood not fully that theme by which the Children entered into the Music, none of the Ainur dared to add anything to their fashion.

The pattern should be obvious: the nature of the Children of Ilúvatar is not for the Ainur to interfere with and doing so is considered evil. I'll leave the last words to Ilúvatar himself:

Why hast thou done this? Why dost thou attempt a thing which thou knowest is beyond thy power and thy authority? For thou hast from me as a gift thy own being only, and no more...