So far, we’ve kept the dragon fairly well contained, but I don’t know for how much longer.

When two pre-critical masses like the two halves of the “demon core” are brought together enough to go critical, the heat and energy generated by the first large burst of fissions would force the core halves apart again violently, terminating the reaction.

If Slotin had not reacted as quickly as he did however, everyone in that room would have died and not just him, and possibly there would have been serious contamination of that lab for a long time and some more horrible radiation-related deaths and/or illnesses by the time the reaction self-terminated.

A lot of the scientists in the Manhattan Project were the same way: twenty-somethings at the top of their class from the preeminent science programs in the world, vastly arrogant and myopic towards their particular field, and not inclined to listen to anybody. Leslie Groves had a bitch of a time trying to enforce various basic military standards on them. It was like trying to herd cats. If they didn’t want to do something they saw as unnecessarily infringing on their glory, they tended to simply ignore it. And once the “build the unbuildable at any cost” Manhattan Project was over and the nuclear weapons labs fell under massive government oversight, 90% of those brave young pioneers ran for low-oversight university research gigs where they could continue to be cutting edge and risky. A number of them were involved in the horrendous human radioisotope experiments that took place in the late 40s and 50s.

TL;DR, Slotin was probably a genius and most certainly a jackass with a malformed sense of self preservation, but those two traits tend to go hand-in-hand most of the time. To expect otherwise is unrealistic.