Stiles wanted to close his eyes to the sight before him. However, he learned from experience that closing your eyes, just made the vision so very much worse.
We watched the scene of all his family and friends being sexually assaulted for as long as he could, until his body's only reaction was to close his eyes. Closing them only multiplied the men and women raping his loved ones. Not even his deceased mother was immune in this horrendous vision.
Open and they are being cut, close and they are being skinned. Open and they are burned with cigarettes; closed and they are being burned at the stake. The sights were bad, coupled with the wailing screams, the putrid smell, and Stiles was going to lose it. Lose everything from his lunch to sanity.
Open and Peter is running his hand, lovingly over his face; closed and the man is behind him. One hand forcing him to see the vision in front of him, the other running over his nude frame as the older man leans over and whispers into his ear… "Welcome to my nightmare, Stiles. I'll make sure you like it."