'Quality of Mercy' and 'The Light Brigade'...
...are the two most depressing episodes ever.
***Spoilers***
First Robert Patrick's character Skokes is tricked into giving away a secret mission to an alien spy about an attack plot to turn the tide of a war that the Earth is losing.
Secondly, in "The Light Brigade", Robert Patrick's character is an alien spy disguised as Skokes that tricks the remaining crew of an embattered spaceship, on one last hail mary attempt at winning the war, to not bomb the alien homeworld but bomb Earth instead. Thus dooming humanity.
Wow, I needed a drink after watching those two.