The eruption didn't kill that person, gung-ho working practices and the lack of strictly adhering to something like OSHA killed that person.
He was working around a newly opened fissure without being clipped into something like a safety harness, which I daresay is standard practice in most of the rest of Europe and the US in similar circumstances.
The soil shifted, or he fell. In either case someone directly or indirectly working as a contractor for the government died as a result of unsafe and obviously dangerous work practices.
The government then went into overdrive to push the narrative that these fissures were so dangerous in general that they had to evacuate the town immediately.
That can also be true, but the immediate cause is needless and dangerous expediency.
In the end they fell into shit and came out smelling like roses, because the eruption happened to start a few days later.
The issue of that easily preventable death was quickly forgotten by most (but e.g. [1], in Icelandic, raises so some of the same concerns).
The evacuation order at the time had nothing to to with an imminent eruption, just the supposed general danger to the public from shifting soil and fissures.
Some other eruption in the area was expected eventually, but that it happened so fast came as a surprise.
1. https://heimildin.is/grein/20601/