Battle not with monsters, lest ye become a monster, and if you gaze into the abyss, the abyss gazes also into you.
Mmm...this quote brought to mind one of my favorite books, Moby Dick. In the novel, Ahab sought to kill the giant white whale, Moby Dick but in doing so, he became as much, if not more, of a 'monster' as the whale itself.
Many of us use this truism to create plots or conflicts in our own novels. The protagonist sees in 'evil' and pursues it to 'save the girl/city/nation/world' but while he/she is doing that, they don't realize that they've closed themselves off to normal human interaction, to the softer emotions which could save them from becoming a monster themselves.
In romance, the solution generally involves the protagonist meeting and falling in love with that one person who can bring them back from the brink of monsterhood. He/she will still destroy the monster but without becoming one. And the destruction is followed by a happy ending.
I think that's why I no longer read the literary type of 'kill the monster/become the monster' books anymore. I don't care for the ironic ending or the ending where poetic justice is the final word. I want to see the protag grow and realize what he/she is doing wrong so that they can live a happy and loving life.
It might take the entire novel for the protag to realize it, but love is indeed the answer.