Alastair Thorne: the Pegasus interrogator whose death split two crews

Lieutenant Alastair Thorne was an officer aboard the battlestar Pegasus at the time of the Fall of the Twelve Colonies, and his job aboard that ship was known to everyone on it. He was the Cylon interrogator, and his methods were torture and sexual assault, carried out with the help of Pegasus marines. (Battlestar Galactica, "Pegasus"; Razor)

Who killed Alastair Thorne?#

Galen Tyrol killed him, without meaning to, by throwing him against a wall of Galactica's special brig, where a bolt left exposed in the metalwork went into his skull. (Battlestar Galactica, "Pegasus")

Tyrol and Karl Agathon had gone in because Thorne was raping the Caprica copy of Sharon Valerii, the copy who later took the name Sharon Agathon. Stopping him took seconds and killed him. Admiral Cain arrested both men, and given how she had run her ship since the Fall, nobody aboard Pegasus was surprised by which side she came down on. (Battlestar Galactica, "Pegasus")

Why did the Pegasus crew defend him?#

Because he had saved their lives, or they believed he had. Specialist Gage said about fifty crew members owed Thorne their own survival, and Colonel Fisk counted himself among them. (Battlestar Galactica, "Resurrection Ship, Part II")

Fisk made that clear to Tyrol and Agathon while they sat waiting to be tried. To him and to a large part of the Pegasus complement, the man who had died in Galactica's brig was not a criminal caught in the act but a shipmate, and the two men in the cell were the ones who had done something wrong. Both readings were available to the same set of facts, which is the point the confrontation was built to make. (Battlestar Galactica, "Resurrection Ship, Part II")

The grievance outlives the ship#

Thorne's death carried on doing damage long after Pegasus itself was destroyed over New Caprica. (Battlestar Galactica, "Exodus, Part II")

Gage never let it go. When he joined Gaeta's mutiny, he went at Karl Agathon directly, arguing that Thorne's death had gone unpunished and that Agathon's promotion afterwards had been unearned. That resentment is what put him in the assault on the Agathon family and at Gaeta's CIC station more than two seasons after the fact. (Battlestar Galactica, "The Oath"; "Blood on the Scales")