A player loses a match and his opponent packs up and walks away. While the losing player sits there an stews, he picks up the table number and begins to slowly tear it into pieces. What is the penalty for this? He didn't steal it. Is it unsporting conduct? How severe?

We know that if he gets mad and picks up the table number and takes it with him it is Theft of Tournament Materials.

Now, if he receives Unsporting Conduct for tearing it up, what happens if he then scrapes the pieces into his bag and walks out with them? :-)

He's destroying the TO's property - I'm pretty sure that TO is going to want this player gone. Even if you don't assess an Unsporting Conduct penalty, he won't be in the venue for the next round, and will get Tardiness & be dropped.

Howver, a quick look at UC-Major gives us our first clue:

Commits aggressive or violent behavior that is not directed at another person or another person’s property.

OK, so it's not UC-Major. What about behaviors that are directed at “another person's property”? Yep, we've got a match; UC-Aggressive Behavior:

A player acts in a threatening way towards others or their property.

There's even an example that's fairly close:

A player tears up a card belonging to another player.

So, I'd suggest UC-Aggressive Behavior, Disqualification, and I'm pretty sure the TO will also agree on removal from the venue.