Who: RED Medic and BLU Pyro
What: RED Medic meets BLU Pyro in the tunnels as promised, and tells him about the true horrors of war.
When: Late at night, after hours, following BLU's discovery of his brother's death
Where: The tunnel system underneath the bases
Notes: Lots of exposition on RED's part about his time in the war, and before RED.

( You do not understand how war can warp an individual. )