He just wanted to do it at this point, and as he says in the next quote, he just wanted to end it. While 'it' could refer to L'Manberg, in this specific context, where he's just realised he would die trapped in the box, ending it most likely refers to himself, his life:
“I just- I just want to f… I just wanna end it, I wanna end it. I wanna press that button, man.” - (Wilbur’s Speedy Stream Festival What festival: 28:08, 17th Oct)
I can't simply just show the quote and his expressions here, the clip says it all, it's in his tone, in his movement, his almost agonising expression, the 'ffff', it shows his desperation.
Alivebur was ready to die here, he was ready to press the button and end everything, and that included his life. And this day, the 17th Oct, is the first instance where Alivebur pretty much says it, says what he's planning to do without saying it outright.
This may have not been when he decided he was gonna die (a couple festival quotes suggest he knew he was gonna die after pressing the button), but this was when he realised and accepted it.
one of my favorite instances of Carl Stalling’s musical genius is from Cracked Ice where he manages to SEAMLESSLY combine “The Skater’s Waltz” with “Little Brown Jug” (bonus points for hilarious Tedd Pierce monologue nonsense)