That would be rather unethical. Most of the time we pay for food
before we get to taste or even see it.
I was quite tempted to do that a few days ago even though I was
hungry (and so are millions of kids around the world). I paid $6.50
and still got a small burnt piece of chicken, EZ cheese and a
miserable excuse for "garden salad". In other countries I would've
felt entitled to throw the food into the drain or at the shop
window. I ate it anyway.
If we exclude walking away without paying in protest of really
bad food, then I would finish it unless it's really bad.
Like in your case if the chicken is really burnt black, I won't
eat it for health reasons. Actually in such cases where the food is
unedible (totally burnt, raw, etc) I would have gone up to the
manager and ask for either a replacement or a refund.
Of course after that I won't ever go back to the place
again.