Lore things I’ve discovered from redoing the gladiator quests:

There’s a poison known as Rhalgr’s Bile, which was apparently used to kill an Ala Mhigan king. Not Theodoric, presumably, since his death is known to have been a suicide.

The Corpse Brigade, a criminal organization inspired by a group of the same name from Final Fantasy Tactics, consists almost entirely of former Ala Mhigan Kingsguard knights.

In other words, these former Kingsguard knights are the only ones who know how to concoct this deadly poison that killed a king they were sworn to protect. It’s not difficult to imagine, then, who killed that king in the first place.