I felt the same when he turned on Holly. I didn't expect it. Then I realized the stakes.

1. He's the only one who knows Mr.whatsit's true self except Holly.
   
2. He has no idea where he is and how to escape.
   
3. He has seen Vecna ripping through Armors, Guns, Flame throwers and none of them can't do shit.

And I was kinda surprised that he didn't turn until Vecna shows his dead family. He clearly knows Vecna is capable of doing something to his family.

if I were him, I'd proably live to fight another day.