Let's first go over the many tricks the killer prepared before they committed the crime. First, they destroyed the contact elevator. This separated Nagito and the others in Strawberry House from our group in Grape House. Next, they lured Nekomaru out by himself by turning back all the clocks in the Funhouse by two hours. Additionally, in order to secure an alibi, the killer went to the Strawberry House lounge... ...and set the wall clock's alarm to 5:30 a.m. After finishing their preparations, the killer went to Strawberry Tower with the necessary tools in hand. They obtained these tools from the Octagon, which you can enter once you clear the Final Dead Room... This means...the killer discovered the secret of the Funhouse faster than anybody else. That secret being Strawberry House and Grape House are actually the same building...
On the morning of the incident, Nekomaru woke up and headed over to Grape Tower for a specific reason. There, Fuyuhiko, who was at the lounge by coincidence, witnessed Nekomaru. According to Fuyuhiko's testimony, that was around 5:00 a.m. but... By that point, the killer had already messed with our perception of time... In actuality, Fuyuhiko witnessed Nekomaru at 7:00 a.m. That's also the same time Monokuma Tai Chi begins...Nekomaru went to Grape Tower to participate in that.
However, because the contact elevator was broken, Nekomaru was unable to go to Grape Tower... So he decided to try going to Strawberry Tower. But...the killer was waiting for him there.
With the power of hamsters, they were able to press the Good Night Button on the back of Nekomaru's neck... This forced him to enter sleep mode, rendering him immobile. From there, the killer began preparing to use the Ultimate Weapon...
First, they set the alarm in Nekomaru's chest to 7:30 a.m. so he'd wake up... Then they tied him up with a metal wire, tied the tip of the wire into a loop, and hung it on the doorknob.
After leaving Strawberry Tower, the killer then destroyed the door button to Strawberry Hall. They did this to keep us from entering Strawberry Tower... ...and to keep us from discovering the secret of the building structure that they used to kill Nekomaru. Then, they used the secret Octagon passageway to travel to Grape House... After arriving at Grape Hall, they pressed the button to open the door to the tower.
When that happened, the elevator-liked floor of the tower began descending... ...and Nekomaru's body was still inside, dangling upside-down in midair from the wire. The killer entered Grape Tower to see if their setup was successful... ...and placed a hammer on the floor to look like the weapon, then wrapped a chain around the back door. This was done to make us falsely believe we couldn't enter the tower from Strawberry Hall.
With this, the killer finished their setup and tried to go back to their room using the secret passage... ...so they could craft their alibi when Nekomaru died from the fall. But something unexpected happened. Fuyuhiko, who saw Nekomaru earlier, was still at the lounge. As a result, the killer couldn't return to their room, and with no options available, time ran out... The lounge's wall clock alarm started ringing at 5:30...well, actually 7:30... To avoid a worst-case scenario, the killer was forced to appear in front of Fuyuhiko with the others.
When the wall clocks' alarm rang, that was also the same time Nekomaru was waking up... He woke up while he was still hanging upside-down, so he couldn't help but sway his body powerfully... Originally, the loop of wire was only supposed to slip off the doorknob... But because there was a heavier load than expected, the doorknob ended up breaking. Nekomaru fell from the 4th floor all the way to the 1st floor...
He crashed into the pillar, which decapitated him on impact, and died... The sound of Nekomaru's impact echoed throughout the Funhouse... However, by this point, the killer's plan was about to fail. Thanks to the broken doorknob...and Fuyuhiko... Meaning, the killer is someone who wouldn't have heard the alarm if they were in the Deluxe Room. They also wouldn't have been able to return to their guestroom because Fuyuhiko was at the lounge...
That someone is Gundham Tanaka... I can't think of anyone else but you!