How Dark Forces 2 foreshadowed the dark side ending.

At the end of Level 1, after Jan saves Kyle's life, the duo share some banter:

Jan: (Smiles) What would you do without me?

Kyle: (grins back) I'd be a content old man.

Jan: Somehow, I see you neither being content or old.

At first glance, you would think they're just two friends laughing and making jokes. Similar to Obi-Wan and Anakin's dynamic.

But fast forward to Level 14, after Kyle murders Maw in cold blood for taunting him about his father's death, Kyle is brought perilously close to the dark side. Then Jerec and his Dark Jedi appear with Jan as their prisoner and tempts Kyle to fully succumb to the dark side by executing Jan as well.

Should Kyle choose the dark side ending, he decides to seize the Valley of the Jedi for himself and murders Jan before killing Yun, then Jerec, and snatching the Valley's godlike power and taking Sariss as his servant. Becoming Emperor Katarn.

In Katarn's throne room, he sits upon a throne in a vast empty room all alone. He views the hologram recording of his father saying how proud he is of his son of the man he became. After a moment of conflict, Katarn crushes the recording underneath his boot.

In killing Jan, Kyle ensured she never got to see him become old. And by severing all of his ties to his previous life, he would never again become content either.