The source (cited in my first comment here) lists the "Nedelin catastrophe" of 1960 as the deadliest in the "space" category with a range of 54-300 deaths. As noted in the visualization and my first comment here, I rounded up to the maximum deaths in cases where a range like this were given.

I can see a perspective where this doesn't count as a space accident/disaster because it occurred on the ground before the rocket was launched, but the source categorizes it that way. All the other accidents/disasters in the list also occurred on the ground or in the air below space except for one: Soyuz 11 where three people died due to depressurization in space. I just decided to go with the source's categorization since they all took place as part of spaceflight operations.