Is it illegal to delete someone else's Facebook account?

Girlfriend dumped me, and I'm just a little pissed.

I'm not going to HACK - I know her password, so is it illegal for me to go on her Facebook and delete her account? Thanks.

A link of PROOF would be nice.
Update: How would it be illegal though, if she gave me her password?
Update 2: ...She dumped me because I treated her flawlessly. Her reason for dumping me was because I was too nice. 9 months, down the drain.
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