Why did nobles and monarchs from other countries fear about the French Revolution?

One element that did not sit well with the American Public was the liberal use of the guillotine in France. John Q. Public had begun to see public execution as just fine for the gallows execution of convicted highwaymen, but regicide and women on the chopping block seemed out of the bounds of propriety and better handled out of the public view. This form of populism like "bread and circuses" was a little to much for American sensibilities.