Often the case is when one side feels that by just doing that little bit more they'd win:

If they used POWs in the industries
Slightly more indiscriminate bombings
Blockading neutral ships
Slightly more widespread sabotage
Helping distasteful allies

Since in many wars both sides feel like this, things slowly escalate.

From the American Civil War onwards powers have not been magnanimous in victory as the victors have lost so much they didn't view it as the old wrestling match.

Taking no prisoners either means the enemy will fight to the death - or run away. Look at some of the most successful armies. They generally took a dim view of prisoners. These days the logistics of prisoners can mean that abiding by the Geneva Convention cripples the war effort feeding and housing them all.