What do you think of the idea that Mandon Moore wasn’t ordered to kill Tyrion by anyone, rather that he tried to do so of his own accord as revenge for Vardis Egen?

Actually, that’s something that’s come up a bit on comments to the original piece. 

Here’s why I don’t think he did it of his own accord: from what we learn of Mandon Moore from those who spent the most time with him (i.e, members of the Kingsguard), he had “no friend but his sword and no life but duty.” 

When we see him in person, he’s almost completely emotionless and lacking in affect. He shows no emotional response at all to the news that Vardis Egan is dead – all he says is that he knows Vardis Egan. 

As far as loyalty goes, he laughed at Ser Barristan Selmy getting dismissed from the Kingsguard, so it doesn’t seem to run deep. We learn that Jon Arryn brought him into the Kingsguard, but due to Mandon Moore’s competence rather than any deep friendship or attachment to House Arryn.

Combining those facets of his personality, I don’t see Mandon Moore as someone who acts according to his own desires. If he swung the sword, it’s because someone told him to.