What did he see at te end that was so terrible

It wasn't that he saw something new; it's that ripping out his eyes solved all the problems that had been plaguing him since the experiment began.

He was going on and on about how bad things were for him, and it never occurred to him(until it was pointed out by someone else) that all he had to do in order to end his suffering was to take his eyes out.

As much as he saw, it was ironic that such a simple solution, always right in front of him, was completely out of the scope of his "incredible" vision.

I think what he saw and described as the "eye that sees us all" was meant to be God. It was a painful and terrifying sensation for him, simply because it was too overwhelming, similar to the lab monkey's fatally intense experience in the beginning of the movie.

He saw Azathoth (MANA-YOOD-SUSHAI for all you Dunsaneans) at the centre of the universe.

"Nothings gonna change my world!"

oprah winfrey in the shower

"What was it?" more like. Lack of sleep, extreme stress, desperation, unable to handle the x-effect, delirium. He was blatantly giving himself up at the casino telling others the dealer's cards. All he needed was a simple suggestion (take your eyes out) to send him over.

"Sorry. If I've not responded to you either it wasn't necessary or I've set you to Ignore."

Here is a link a quesitonnd sw nswer about the different endins in the movie and the comic book based onit.

https://scifi.stackexchange.com/questions/269430/the-comic-book-version-of-x-the-man-with-the-x-ray-eyes-1963