[l3x4ur1n]

I mean, if this should happen to me, I want to undergo euthanasia as soon as possible. If I am already dead, I don't want to unnecessarily suffer. So my question is, did he not want the euthanasia or was it not "accepted" or why he had to suffer so much?

[fwip]

The first person the demon core killed, Harry Daghlian, notably allowed the doctors to study and record information about his deterioration due to radiation. I believe Slotin had a similar motivation - that at least, even this slow, painful death could provide valuable information to doctors and scientists.

[rob74]

This was the United States in the mid 1940s, I doubt euthanasia (or even assisted suicide) was much of a thing back then. Plus, as someone else mentioned, there was also the scientifical aspect of being able to study the effects of irradiation.