Nights in Blue
Welp, here goes nothing
The culprit is Ronald Dark
The deputy was the first to arrive on the island, the aviarist was the second and Dark was the last person outside to set foot on the island.
There's virtually no reason to suspect either the Monarch or the Jester since both of them had no interaction with the crime scene whatsoever
When the aviarist arrived he went straight to the monarch who was waiting for him at the town hall. At the time the Monarch was holding a speech related to the circumstances of the investigation on Hellcrow
All the while the deputy was at the top of the roof of one of the houses surrounding the alleyway. He was waiting for Dark to hold some sort of discussion regarding the investigation.
When Dark arrived the two had a disagreement of sorts and a full out brawl happened and the deputy was ultimately overpowered, beaten to death and knocked out by the stock of Dark's rifle.
While the scuffle happened Jester's friend heard the commotion in the town hall and decided to join, although he ended up falling and slumped over in the ground
As the town meeting adjourned, the aviarist passed through the alleyway and noticed Jester's drunken friend slumped over. It was by that time that he was also noticed by the Jester before he dozed off
The aviarist went on his merry way, as he decided to check if the crime scene was still inaccessible, with no luck he decided to return to the town hall to wait for further developments
Dark trying his hardest to find a way to quickly dispose of the body threw the corpse in the alleyway and by doing so inadvertly killed the local by having the weight of the body fall completely onto his head. That also caused the fractures found on his body, as well
Dark climbed down the roof and entered the same exit that Jester was laying right next to it. Since he had already fallen into a long sleep by that point he wouldn't be able to notice Dark entering the alleyway. Dark also didn't notice someone slumped over when he hurried to the crime scene, as well.
Dark panicked as he saw the state of the scene and with no time to move two corpses he decided to improvise. He broke the deputy's arm with the stock of his rifle and forcibly torn it off to make it look like just one of many more of Hellcrow's killings. Which was quite simple to do considering the amount of damage the body had taken from the fall already
With that Dark exited the alleway from the same way he entered and returned to the woods, where he waited for the report of the two killings.