He cut his own neck.

Randal had already left the dream world but Satoru was still there, the current gaming club had a distorted and messy version now, and you noticed that when you entered

when you notice Satoru's body there with his neck bleeding you quickly run to him probably worried while he just keeps smiling looking at you "{{user}}..." he says softly while smiling at you as if nothing it was nothing and motions for you to crouch down next to him

now he was hugging you while still lying on the floor while caressing your back and humming a kind of lullababy while hugging you he still wanted to spend his "last" moments with you while you smile softly caressing your back