...as night and day.
Black, and White. Evil vs Good.
Those two posts could not delineate the chasm any better.
And in some dark chamber of your heart, some closed room in your mind that you have been trained to keep locked and shut because, well, you're civilized, aren't you? Well...
You know who needs to be killed. That they need killing, for you and yours to survive. And why not...
They'd do the same for you.