I figured he might’ve been stabbed once. But, 22 times...
He should be in a secure location. They knew other prisoners would try to kill him.
Whether one can be in protective custody would depend on whether there is someplace to put him. He may have decided against it, we don’t know on that.
However, you are thinking as a normal person. Normal people don’t go into stabbing frenzies to ensure death. The inmate in question obviously wanted to kill him and tried to ensure that would happen before he was stopped. Somebody in a frenzy can do 3 or 4 stabs every second and it takes time to cross a room to stop somebody because the guards can’t be everywhere all the time.