While I also agree, I think we must take into consideration that the game came out a long time ago.
Back then, the prequels weren't released. We only knew about 2 Siths - Vader and Palpatine. Palpatine was basically a completely corrupted individual who cared about nothing else than power. Vader had more depth, but much of his backstory was unknown at that point. Also, there weren't many EU books/things to draw from.
So I think that it's a pretty good indication of how the Dark Side was conceived at that point. The designers wanted a powerful scene to show Kyle's fall, and I guess that the best way they found was to make Kyle kill his friend.
~Dark Cupcake of the Sith