@dimasmw What you wrote would make sense if he actually did some thinking but he doesn't. He is not being rational. He is being cowardly. He is afraid of what awaits in the game that he refuses to return.
If he dies in game, he dies in real life. This is the most important information in the manual. Just before being logged out forcefully, he had a quest that will kill him if he fails.
@Qenit
I don't think you understand what the word 'directly' means. At most, he indirectly caused the person's death. He didn't kill him. He just didn't let him win.
@Huichelaar
The souls of the dead students and teacher were going to end up in isekai because of the crack in space. All D did was allow them to retain their memories.