I guess it was an accident, but this kinda shit alienates countries that are best lackeys of the US. Perhaps on paper ofc, they have to do the lip service where necessary, but still.
Israel has the sharpest technology, they can pick targets, and execute properly. I could attribute the strike to malice, or perhaps they had intel that Hamas hiding there or having a c'n'c base there. But everything cannot go flawlessly, so these thing will happen, especially at a place so crowded as Gaza.
And many people did not leave, and some who left returned. IDF did airstrikes and bombardments on the south too, so what's the point of leaving if they can get hurt elsewhere?