Kyle's father, Morgan Katarn was slain by the Dark Jedi Jerec, while he was the last person in the galaxy to safekeep the location of the Valley of the Jedi. Jerec wanted more power, more than anyone could handle.
Kyle was formerly a mercenary, working for the Rebellion, and ifiltrated the Empire in Dark Forces
. His father's death was a tragedy and Kyle wanted to avenge it.
That's about as far as I can go on the "Unknown History" part. As for his death, that did not yet happen.