Sean was part of a project which was experimenting with warlock. He tricked Jack into bringing Ace and Shell to the lab, since the project wanted to start testing the drug on human subjects.
After their minds were transferred into the bodies of animals, they escaped but Shell was caught and killed. Shell's human body briefly reanimated and throttled Sean to death. (PROSE: Warlock [+]Andrew Cartmel, Virgin New Adventures (Virgin Books, 1995).)