Someone said to me not long ago that they felt there was no book of man they could not fathom easily. There are a few things happening in Gene Wolfe's The Wizard Knight that put the lie to that.
The mystery of King Gilling's murder is a subject that will be of continuing interest to Wolfe scholars. In his immortal guide to Wolfe's book, Michael Andre-Driussi doesn't venture to tell us his own opinion. We can infer that he thinks it is more exciting to come to the conclusion on our own.
Mr. Andre-Driussi provides us with most of the textual evidence on this question, rather like a CSI technician passing along the clues. Wolfe's best detective, of course, was the narrator of "The Detective of Dreams." The double meaning of this story's title was that the title both referred to the narrator and to God himself, who was doing the dream-making. His second best detective is Mr. Blue of Pandora by Holly Hollander, and he too makes much of the mischief, both bad and good, in that novel.
The entity most interested in the answer to the question of who murdered King Gilling - and therefore the detective of The Wizard Knight - is Thiazi, one of the most enigmatic characters in the book. Thiazi appears to have changed from his native horrible giant state to a creature of different ideas, perhaps because of the love he naturally lost.
From what I've just said, it would be fruitless not to debate the question of whether Thiazi himself was the murderer. There are a few major points in his candidacy as the killer:
1) He suggests the same person attempted Gilling's murder during the melee, and then killed Gilling in the courtyard during the fighting at the gate to Utgard. This indicates his guilt, because if that is true, then he is the most likely candidate for the killer. As he notes, he was present at both incidents.
2) Able will not tell Thiazi who the killer is. Since Able is usually honest, especially when he is begged, this does not look very good for Thiazi. On the other hand, most of the other potential candidates also have reason to be protected by Able.
3) After Schildstarr ascends the throne, Thiazi is right by his side. Able explains this as a political calculation, but it is far from obvious. A strange comment by Able after seeing how the giants have rearranged their government is a muted tip-off that the intrigue in the castle is not all that it appears to the humans. There is even the implication that Able and Thiazi have essentially accomplished a coup, since it was impossible that Gilling would have allowed Idnn to leave Utgard.
4) Schildstarr appears to be a more powerful king than Gilling, and may even be controlling some of the giants he purports to fight for the king against. His sorties against Able's camp aside, he appears to be a better ruler and this would benefit Thiazi.
6) The fact that Thiazi is unable to find out who the killer is indicates the killer could conceivably be him.
But let us review the pertinent facts briefly once more.
Org kills six giants during the melee, but doesn't have a knife and didn't stab the King. He is capable of moving extremely quickly over short distances and could have turned off the lights before the incident, but it equally could have been Baki. A faux giantess appears - this could be Baki, but we don't usually the see the Fire Aelf adjusting their size in that fashion. It is more likely Huld, who can assume any form.
The manner of the attack during the melee on Gilling indicates the perpetrator as well. The person was either of a size to wield the stabbing implement, or wanted it to look as if they were. This points to Thiazi. Perhaps he intended to make it look as if Toug had done it.
Svon says that he would have done it, but he indicates he didn't. This would be a difficult task for him, because his right eye has already been swollen shut. He certainly didn't actually kill King Gilling later on - he was fighting giants at the time. The same is true of Garvaon. In the fighting, he may have thought that he had - he slew giants during that unfair battle. Thiazi even goes out of his way to praise the valor with which Garvaon and Svon fought.
There is also the issue of Thiazi's size. Able describes him as three times the size of Beel, but Beel is a tiny man. He's smaller than King Gilling, and that would be about the height of the stabbing in the lower back.
Then there is the question of who is giving Thiazi instructions. For that we turn to the Valfather, for he clearly has a dog in this fight besides Able. He has informants in Mythgarthr, and the fact that Thiazi possesses Able-like powers is an indication that he is one of them. Huld may be as well. They may even toss Gilling to his death together, seeing that it was the perfect time for the King to go away for good now that his kingdom was falling apart and so clearly needed new leadership.
What is the Valfather's motivation to strike against King Gilling? I believe it is to preserve Queen Idnn, who he regards also as his servant in Mythgarthr, and who is the namesake of the goddess.