The challenge isn’t finding counter examples, it’s limiting the narrative of history to have a simple “good/bad divide” but here are some selections one may consider the underdog the greater of two evils:

• The confederacy (US Civil War)
• Caesar’s army crossing the Rubicon
• Imperial Japan post Pearl Harbor
• Pakistan’s complicated history with India
• King Leonidas and his men (Persia was arguably more equitable)
• Austria-Hungary (WW1)
• Fascist Italy (WW2)

As for David and Goliath, keep in mind David was using a sling, a weapon that can hit with the same force as a modern revolver when used properly. It’s entirely possible that story is an allegory and/or propaganda of the value of properly equipping one’s armies; it probably shouldn’t be viewed as anymore impressive than a rampage killer fatally wounding somebody twice their size