Darkness or Light? Philosophy in Knights of the Old Republic.

Why... there are definitely moments of mercy among the Sith... KOTOR 2, again, is undeniable proof... Darth Nihilus... spared the last representative of the Mirialan species, whose planet he destroyed...

A funny argument, considering that when destroying a planet with billions of inhabitants, he spared ONE. That's the height of mercy. I haven't played KOTOR 2, but if you say there are interesting conversations with companions, then I recognize the hand of a master:)

Why - the Sith may be evil, but they are not beasts... they will understand...

-"If you order your old and loyal servant to carry out a task and he fails, will you spare him?" - Uthar Wynn
-Yes - Hero of KOTOR 1
-And thereby show your weakness! Go, you don't understand the Sith Code!"
And also, about love:
-"Is it true that love is the worst thing?" - Uthar
-Yes - Hero of KOTOR 1
-That's not true! Mercy is worse than love (implying that love is slightly better than mercy).
The Sith, in my opinion, recognize not love, but passion, and these are by no means the same thing. Love can prevent a person from achieving power, which is the Sith's top priority, while love implies that the number one goal is oneself, and not something else.