41
You could drop a comet on Mars without doing much harm, because there wasn't much on Mars to do harm to.
The land surface of Earth, on the other hand, was vulnerable at every point. There was hardly a square kilometer of land left there that didn't have some human beings living on it, or building on it, or depending on it for something they needed for their lives, even if it was only forest cover that they needed to keep some otherwise useless mountainside from eroding down to choke their water supplies.
The impact blasts from the comets were only fireworks on Mars. On Earth they would be Hiroshimas or Colognes if they impacted on a built-up land area . . . and even if they struck the empty sea their impact would produce the vast, spreading ripples of ocean that would overwhelm the first coast they came to in the form of tsunamis—tidal waves—bores of water that would sweep away coastal homes, and drown buildings and people. A single comet fragment striking the Earth could kill millions.