They weren't that cavalier, else, for example, a lot more people working on the project, especially at the fissionable production plants, would have died from radiation exposure, instead, it was just these two working on the Demon Core.
This feels more like arrogant scientists believing the rules didn't apply to them, that particularly true for Daghlian, who was for example violating the very sensible rule that you don't work on this sort of thing alone (e.g. a coworker might have pointed out the danger of the experiment, or independently taken more effective action to stop it before fatal dosing happened, e.g. partly in the manner that Frisch saved himself, by knocking aside some of the reflecting blocks instead of Daghlian's attempt to remove the block he'd dropped).
(See also the Los Alamos team of junior scientists who violated both direct orders and all common sense in trying to whip up their own smoke pots, resulting in as I recall one death and one blinding. I knew better than that when I created my own pyrotechnics in high school (granted, I'd been on the track to become a scientist since 1st grade, but...).)
Seriously, Slotin should have either realized the danger if his hand slipped, or should have realized that his hand slipping was too great a risk for really no serious gain.
