From Team Fortress Wiki
Voice Responses are contextually triggered lines that play after the player has achieved something, for instance killing a certain amount of enemies with a Primary or Melee Weapon, or the player has triggered something, like being set on fire. The Demoman has several vocal responses, all of which are listed below (excluding Voice Commands).
Kill-related responses
Event-related responses
Weapon-related responses
Objective-related responses
Note: Most lines spoken by the Demoman Robot are robotic versions of the lines above. The following are lines that possibly hint at the Demoman's and the Soldier's friendship from the War! Update.
During battle with Merasmus
Miscellaneous
Duel-related responses