The empty weight of a Falcon 9 first stage is somewhere in the neighborhood of 14 tons. It would be vaguely comparable to a DC-9 crash with little fuel left in the tanks. Not quite as bad, as it's lighter (DC-9 is about 22 tons empty) and considerably less dense. So it would be a "take out a couple of houses" event if it hit the wrong spot, I think, but nothing much worse than that.