It killed <75 people.
They blew up the dam because it was also a bridge and removing it allowed them to redeploy troops to the east.
But the thing about dams is that they're over-engineered and massive. Even with high explosives it's difficult to demolish one. Hence why dam busting bombs were large (>4000 kg) and still required underwater detonation to boost their power [0].
It wasn't unreasonable for the Russians to expect Ukraine might be able to seize a bridgehead on the far side, rebuild an operable road over the dam, and then rush armor (and logistics) across it.
Much harder and more time consuming when there are no longer any load bearing remnants available.
[0] https://en.m.wikipedia.org/wiki/Operation_Chastise#The_attac...
It was, as much as it was unreasonable for russians - that's why they ran over to that side of river.
There's a reason Russia blew up the Kakhovka hydro dam. The down-stream impacts of the flooding were more devastating than what they could reasonably accomplish with conventional weapons.