I have theories for each suspect. Here goes.
Cyrus: Once the mayor put the hotel in place, no one came to the recycle shop anymore. He and his wife devised a plan to murder the mayor.Reese bought the Axe from Leif, and then gave it to Cyrus. He murdered the mayor, and then ran away. Over time, the villagers began figuring it out, so Cyrus had to off them.
Tom Nook: Tom was mad nobody bought houses anymore in moonlight. So he went to the man responsible: the mayor. He and the mayor got into an argument, and tom ended up beating the mayor to death. Villagers found out, so he offed them
Leif: All the visitors in moonlight had trampled the plants in town! Leif had to have revenge. So he started with the mayor, and the then murdered every visitor to the hotel.
Copper: He couldn't clean the town up from dangers to the public! All the hotel visitors made noise and partied way too hard. So he grabbed an Axe and practiced police brutality on the perpetrators. He his the evidence at the station.