He gets better from there, but he never gets completely well. Even into fourth season, it doesn't take a lot of stress to have him go manic. So one of the most brilliant things they did last night is to show that that was a necessary part of the story. Because a John who wasn't damaged would never have conceived the attempt, and a John who hadn't shown near-insanity at the peace conference would never have been believed by the Emperor and Grayza. But they had seen him walk in with a nuke on a dead man switch slung over his shoulder, and seen him _use it_.
And again, I loved how ambiguous they made it. I thought it was so perfect and telling that as he collapses and Aeryn screams agonizingly for help, all the others hesitate. Because they may love John, but they've just seen him throw open the gates to hell, give away all their lives without asking permission.
On edit: I also found it fascinating that Einstein was apparently waiting for him to reach that point. The first 'time...' dialogue ended on 'time is... relative', that John comprehended the dangers of wormholes to change time. Here, 'time... ends'; John understands their apocalyptic potential, and _then Einstein gives him the key_.
ETA: My favorite line from a review (on televisionwithoutpity.com):
Also loved Aeryn's line "Oh NOW you cry." It's like, "yeah Mom, all I've heard was shooting the whole time I was in the womb. The silence is scary. Please tell me more about how to load to weapons."