It's a pity they both can't lose.
— Henry Kissinger, on the Iran-Iraq War
Yugi: Look! Kaiba's dueling Pegasus!
Joey: But they're both villains! Who're we supposed to root for?
Cybermen: Our technologies are compatible. Cybermen plus Daleks: together, we could upgrade the universe.
Dalek Thay: You propose an alliance?
Cybermen: This is correct.
Dalek Thay: Request! Denied!
Cybermen: Hostile elements will be deleted! (They open fire... to no effect)
Dalek Thay: EXTERMINATE!
—Doctor Who, Doomsday
Angry Joe: Sometimes the best way to deal with a madman is to send in another madman...
Linkara: (thinks for a second) That is a stupid plan!
Angry Joe: Exactly! A stupid plan for a stupid man!
Linkara: Are you high?
Demon: You cannot hope to defeat pure evil!
Slade: Actually, I'm not such a nice guy myself.
When there is evil in this world that justice cannot defeat, would you taint your hands with evil to defeat evil? Or would you remain steadfast and righteous even if it means surrendering to evil?
— Lelouch Lamperouge, Code Geass
Why the Hell is [the Obviously Evil demon] slaughtering us for? WE ARE THE BAD GUYS!!!"
Have you ever heard a foolish old saying that evil can only be conquered by good? It's a lie. Evil can only be conquered by evil.
— Welstiel Massing, Dhampir
Only through victory can the galaxy be saved from extermination, so that it can be enslaved, and oppressed, as is proper.
— Galactic Civilizations II: Dark Avatar intro
When evil fights evil, good triumphs - usually. In the battle between Vecna and Kas, though, the ultimate victor might have been evil. While the battle destroyed a particularly loathsome empire, it also resulted in the creation of three powerful artifacts.
You think that there are the good people and the bad people. You are wrong, of course. There are, always and only, the bad people, but some of them are on opposite sides.
— Patrician Havelock Vetinari of Ankh-Morpork, Guards! Guards!