Do this Sherlock style . . .
Motive
Opportunity
Means
Motive . . . well, lots of folks have motive as has been discussed here but IMO the big ones in order would be Katherine, followed closely (or maybe surpassed) by Sun Tzu and finally Blakist. The Blakists I think have the weakest motive since its just 'chaos' but this is a point where the Blakists are working to get into the Star League and see it as a sign of their divine plan.
Opportunity- ComStar, covert Blakists in ComStar, AFFS staffers of his command, LIC/Loki
The poisoned drink was in his cabin. IIRC the zero-G bulb also showed the traces, I do not think they could point to it being either the bottle or the bulb where the poison was introduced. So, it would have had to be someone who had or could fabricate legitimate business on Truth and to be in the flag quarters section. I do not think he had guards posted at his door (which would be why old sailing captains & admirals had marines posted at their doors) so anyone who could find themselves in the area could make a run at the door locks. And if they got that far they would have the tools to enter the cabin. This gives us the first three in order.
LIC/Loki . . . if the poison some how made it from IIRC Glengarry all the way to the battlecruiser without being tested it COULD have been poisoned at the distillery or enroute to Morgan. IMO this is extremely unlikely (hello, food tasters!) as there is a reason nobility has been testing their food since poisons entered political discourse. But its BT where practical actions do not always happen . . .
Means- Who has the ability to pull off such a murder?
Word of Blake, ComStar, Sun-Tzu, Katherine
As the super-able bad guys of the time, the Word of Blake had all the high tech toys- ability to falsify credentials, electronic door picks, ROM's old databases with perhaps security overrides for ComStar ships, and methods of defeating seals on containers without leaving a trace. ComStar ROM could likely lay claim to the same things but they were shown to be a tad inept after the best left for Word of Blake. Also, both Word of Blake & ComStar would have the easiest time of putting together a action cell in Serpent or even on Truth unlike other possible actors- multiple agents attempting the assassination presents its own complications compared to a individual effort but it comes down to the plan.
Sun-Tzu would have the chops for a single agent effort IMO. In fact, being the Sun Tzu and having access to the communication and planning of the expedition would also give any assassin he sent a slight advantage over Katherine or anyone else outside of ComStar/WoB. And while we do not get a lot of looks at how Sun-Tzu has used his covert operations, we do have two glaring examples . . . Loren Jaffray was sent to disrupt the Highlanders' place in the AFFC and the 2nd Blackwind Lancers CO who was a deep cover operative. If you consider his motive strong enough to be playing the long game of weakening the AFFC, then it DOES follow those operational patterns.
Katherine has relied (or over-relied) on covert operations to allow her the public face of the peacemaker. Her propensity for assassins to solve problems is also well known so it fits her behavioral pattern. A enemy (by her definition) was out from behind much of his security in a different environment with a lot of 3rd party individuals present . . . which is a opportunity with a higher chance of success than trying to kill someone on their home grounds (Hello George). In fact, I think that she had a Ops Plan in place to end George on New Syrtis is a good indicator that she had Morgan on her 'to die' list.