And then the Metrons go and more or less act like really arrogant humans who just felt like having fun by having two "lesser beings" play Celebrity Deathmatch.
Instead of coming off as "Incomprehensible Aliens", the Metrons just come off as jackasses who were bored and wanted some bloodsport.
I see nothing in the episode that supports that interpretation. Here is the entirety of the Metrons' dialogue from the episode
METRON [OC]: We are the Metrons. You are one of two crafts which have come into our space on a mission of violence. This is not permissible. Yet we have analysed you and have learned that your violent tendencies are inherent. So be it. We will control them. We will resolve your conflict in the way most suited to your limited mentalities. Captain James Kirk.
KIRK: This is Kirk.
METRON [OC]: We have prepared a planet with a suitable atmosphere. You will be taken there, as will the Captain of the Gorn ship which you have been pursuing. There you will settle your dispute.
KIRK: I don't understand.
METRON [OC]:You will be provided with a recording-translating device, in hopes that a chronicle of this contest will serve to dissuade others of your kind from entering our system, but you will not be permitted to communicate with your ship. You will each be totally alone.
KIRK: What makes you think you can interfere with
METRON [OC]: It is you who are interfering. We are simply putting a stop to it. The place we have prepared for you contains sufficient elements for either of you to construct weapons lethal enough to destroy the other, which seems to be your intention. The winner of the contest will be permitted to go his way unharmed. The loser, along with his ship, shall be destroyed in the interests of peace. The contest will be one of ingenuity against ingenuity, brute strength against brute strength. The results will be final.
KIRK: Just a minute
METRON [OC]: There will be no discussion. It is done.
METRON [OC]: We are the Metrons. Your Captain is losing his battle. We would suggest you make whatever memorial arrangements, if any, which are customary in your culture. We believe you have very little time left.
MCCOY: We appeal to you in the name of civilisation. Put a stop to this.
METRON [OC]: Your violent intent and actions demonstrate that you are not civilised. However, we are not without compassion. It is possible you may have feelings toward your Captain. So that you will be able to prepare yourself, we will allow you to see and hear what is now transpiring.
METRON: Does my appearance surprise you, Captain?
KIRK: You seem more like a boy.
METRON: I am approximately fifteen hundred of your Earth years old. You surprise me, Captain.
METRON: By sparing your helpless enemy who surely would have destroyed you, you demonstrated the advanced trait of mercy, something we hardly expected. We feel there may be hope for your kind. Therefore, you will not be destroyed. It would not be civilised.
KIRK: What happened to the Gorn?
GORN: I sent him back to his ship. If you like, I shall destroy him for you.
KIRK: No. That won't be necessary. We can talk. Maybe reach an agreement.
METRON: Very good, Captain. There is hope for you. Perhaps in several thousand years, your people and mine shall meet to reach an agreement. You are still half savage, but there is hope. We will contact you when we are ready.
There is not a single word there which suggests that the Metrons themselves are taking enjoyment from this.
It looks to me more like they were applying a rather unusual variant of the Prime Directive. "We disapprove of your primitive violence, but we respect your cultures' right to choose your own course, so we'll let you play out your violent confrontation, but we will mediate the contest and arrange things so that you can quickly arrive at a decisive conclusion with a minimum of collateral damage."