Whoever fights monsters should see to it that in the process he does not become a monster. And if you gaze long enough into an abyss, the abyss will gaze back into you.
Source: Beyond Good and Evil
In fiction (and in real life at times) people embrace the inner monsters (sometimes literal, sometimes figurative) to gain the power they need to defeat their foes, get to the next level, or grit through something they'd rather not do. If this is something that seems to work, why are we specifically warned against it?
In any war, in any problematic situation where horrible things are happening, it can be impossible (or virtually so) not to use the selfsame tactics of those that oppose you. The saying goes "ALL'S fair in love and war" and even if you'll pay for doing it in the long run by being seen as a monster, by doing these horrible things (or at times even MORE horrible than those that oppose you), you'll have the satisfaction that you won, even though at day's end it was a morally a pyrrhic victory. But if you accomplish what you set out to do, sometimes it's worth it. But such things can't help but drag you through the soil of morality, dirtying you in the process.