Both can be simultaneously true. Slotin was already regarded as impatient and reckless before said accident - perhaps these types of missions tend to fall on those predisposed to take them.
I had a near miss in my 20s in a research context. In retrospect it was very close. It was really unimportant research. I think its important not to assume too much intent here on behalf of Slotin. He might have been angry or jubilant or hungover or disatracted.