Why would Liao want him dead? Sun Tsu was First Lord at the time, if the mission failed it reflected on him.
Victor ruled out Kathrine primarily because "she wouldn't be that stupid" but we already knew that she was even stupider than that when it came to getting back at her brother for perceived slights.
I won't speak for Guardsmen, but SunTzu wouldn't have had any trouble putting it on those on who were or should have been on the mission, the SL was a means to an end for him anyway, preserving the enemies of his enemies while getting a less exp march lord, and potentially fueling a rival's civil war with the added benefit that up against the clans is one of the few places a Hasek could drop dead without the name Liao coming up seems like a bargain. In fact the more I think about it if Sun-Tzu didn't do it he was probably just pressed for time ;)
I'd have to go with the woblies, prehaps in more of a old school C* way, for most of the reasons above, however I think the writers were more concerned with an interesting plot hook then an overarching setting storyline.
Morgan Hasek-Davion was a competent AFFS General, he was personally loyal to Hanse Davion, and the rightful leader of the Capellan March. All three are ample reasons empigj for a Chancellor to arrange his assassination. What you seem to forget though, is how little chance Task Force Serpent was given. The First Lord assigned Kingston’s Rangers, late of the Chesterton Reserves, to the mission. They were expendable and this was viewed as a suicide mission. If anyone survived, and actually returned, it would be viewed as a bonus. The Great Refusal wasn’t part of the planning stages, it was pretty much off the cuff when Victor and others joined up and sensed an opportunity.
So any failure on the part of Morgan Hasek-Davion would not be a reflection on Sun-Tzu’s leadership. In fact, by sending a Davion Commander, he deprived his greatest enemy of a strength, and showed himself to be above historical conflicts. There were other capable leaders in the Task Force, and while I do not think Sun-Tzu was behind it, I wouldn’t put it past him. He had the means, he had the motive, and he had the opportunity.
George Hasek was young and inexperienced. He also was a pale imitation of his father. With Morgan’s death, the effect on the mission was negligible, but a powerful enemy would not be returning, and his replacement was a lesser man. By killing him before the Task Force made contact with the Clans, it also robbed House Davion of any glory associated with it should they prove successful. Killing him afterwards might have created a martyr. Killing him before created only intrigue.