Because responsible nations are supposed to be more restrained and offer measured and targeted responses than terrorist organizations.
The Israeli government has killed somewhere north of 8,000 civilians in less than a month. That sounds like a big number, but it's worse than that when you put it in perspective: over the course of nearly twenty years the US government killed around 200,000 civilians in Iraq, which means in less than a month the Israeli government has killed nearly as many civilians as the US did per year in Iraq.
That's just crazily disproportionate.