(This is just a fictional story I created, based on Sandbox party plays that often results in such bad endings...)
There was a tank whose username is Kant. He plays on Sandbox when he gets bored or frustrated after playing on other game modes.
However, the arena is so small that he can't even feel the joyness of Sandbox. He neither can't test some of his own builds nor feel some thrill while fighting bosses in his server, even without God mode on.
After almost a month doing this, he thought of an idea that may make him joyful. Through use of such social media conversations, he invited his friend by giving them the server's party link, along with a set of his rules. All of friends from different websites get his invitations. However, unknown from his mind, one of them, with the username GT112, is a pure Diep.io troller and rulebreaker, popular after making several other tanks angry against him. He invites other trollers on Kant's server to ruin the game.
After seconds of sending invitations, Kant later noticed that the arena gets bigger, with a username after him in the leaderboard. More of his friends later joins except GT112 and his gang, and all are playing fairly as what on his rules are. Kant feels enjoyment as he shoot and ram other tanks.
After half of an hour, GT112 had gathered the troller gang, and started to join in Kant's server. At first, they try to act like normal tanks. Until Kant loses attention on them, they quickly switch to Arena Closers, and start the chaos.
Kant, far from the trollers, later see some extremely fast bullets travelling down. He thought at first they were just Rangers, but when one hits a Spike tank and destroys it, he was got shocked. He manages to find where the bullets came from, and when he saw GT112's gang, he was about to shoot when one of the trollers fired a deadly bullet on him.
When he respawns, he quickly switches to a Dominator and pursuits the trollers, but the bullets' knockback, combined with his slow movement, prevents him from catching them. Other tanks also switch to special classes, and later break his rules.
After failed attempts of stopping the chaos, Kant has no choice but to leave his own server. He felt such more frustration than before. He then stops playing Diep.io for a while.
On the other hand, GT112 and the trollers are still making chaos on the server, but they never noticed Kant's leaving. They were got surprised when a notification saying "Arena closed: No players can join" pops up. When GT112 was the last survivor from the real Arena Closers, he managed to turn God mode off, but later was still kicked from the server.
After hours of not using Sandbox, Kant got bored again after a Maze play. He returns again in Sandbox, and gives party links in his friends. But this time, he carefully chose who can only join his game. When all of the invited-only friends joined, they got so much fun that the server lasted for over 6 hours.
After such events, Kant learns that he must choose the friends only he totally knows and can be trusted.