He was trapped.
Well, both physically and mentally, as he had nowhere to go, and no friends to comfort him in his last hours. He just had to be the one trapped in that capsule, didn't he?
In all honesty, Zimsky wished for Josh to be the one trapped. He wanted to make it out of the mission alive, and it was clear in his own intentions. The fact that his struggle for survival had caused him to go berserk (and earn a punch in the face from Braz) just emphasized his desperation. However, he knew that it wasn't the right thing to think about.
This man, this very same, old-looking yet actually young man… He was wise. He realized how much damage his ego had done to his life. To his relationships. To his work. The same work which stopped the Earth's core from spinning, and he wasn't hesitant on blaming himself. Besides, the bomb that kept him company was ticking away, and he'd go down with it.
Zimsky heard Braz's voice, as if it were distant… But he could not make out the words. It was all a blur, and the space enclosing him was getting hotter and hotter due to the lack of insulation, which wasn't helping his nervous cause. Instead, he went through past memories. The days where he had to struggle with his schizophrenic father, the days where he became friends with Edward, and the day he lost him due to the dispute over him stealing his own friend's research, the days where his mother had passed due to cancer…
Was he ever content? Not really. Ask him what he thinks of himself, and he'd probably say "I'm the best!" which could actually mean "I'm a perfectionist and it's slowly killing me".
Thirty seconds. He figured to give a final recording of a speech.
Twenty seconds left, and he stops. A smile creeps onto his worn and weary face. "What the —- am I doing?"
Ten seconds.
Five.
He laughs. He laughs not just because of his stupidity, but at the fact that he'll now be free. He'll see everyone again, and he'll finally be able to apologize to his long-lost friend.
One.
The blast completely disintegrates him, but it's quick. Painless, at most.
Everyone considered him as one of the four heroes that sacrificed themselves for the sake of humanity. What would we call him? Just an ordinary citizen who needed to put his life together…
From this, he finally did his job.