Bittersweet is usually a win at a high cost.
You may have defeated the big bad, but you lost your love interest/best friend/parent/etc. It's not a nobody wins option, cause the hero still accomplishes his goal and it doesn't mean the bad guy "was right."
Look at Gears of War 3's ending:
They finally manage to destroy the Lambent, which in turn puts heavy damage to the Locust, but at the cost of losing Fenix's dad and best friend, Dom. Doesn't mean the bad guy was right, just that the cost of victory was high.
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TheRock ~ Slow dramatic zoom-pan. Doesn't phase the hooded man.
"You have issues." - MWC. Pot. Kettle.