Board Thread:Questions and Answers/@comment-31677332-20181226215623/@comment-27345308-20181226215926

You can only be on Genocide Route if you really wanted to be.

Undyne can't be spared. You have to run from her through the gate to Hotland. After fleeing three times you will be able to immediately flee from her at the start of the battle. After she collapses in Hotland, bring some water from the water machine to her if you really wanted to "spare" her.