On the one hand, we have air strikes, drones, artillery and what not - easy kills from a distance.  But on the other hand, pretty much anyone can put together a rather nasty bomb, and an AK-47 can turn just about anyone into a soldier.  

That wasn't the case in the medieval and early modern worlds, when weapons were hard to use effectively without training and experience.  It was harder for a popular resistance to be effective.

Another key difference in the modern world is the ideology of nationalism.  On average, ordinary people seem to care more about who their rulers are than in the past, and are much less likely to tolerate a foreign conquering power.  

But more to the point, "kill everyone" only makes sense as imperial doctrine if (a) you have surplus population in the imperial center that you want to resettle, (b) you have surplus population somewhere else in the empire, and the cost of killing everyone and relocating a workforce to exploit the resources formerly occupied by the people you killed is less than the expense of bribing or threatening the local strongman to force the locals to exploit the resources for you, or (c) the locals are annoying to the administration of neighboring colonies.

In ancient times, (a) and (b) were fairly uncommon. And I doubt that you'll find any ancient empire which didn't intermittently exterminate particularly annoying colonial populations.

- Jake

Friends come and go. Enemies accumulate.

• Earlier empires weren't any less soft-touch and didn't mind genocides, if only as warning to others. The Mongols 'excelled' in parts of Jin China, Western Xia, Persia (where whole cities of up to one million inhabitants were slaughtered), parts of the Kievian Rus, and of course Baghdad. A memorable episode of the unification of Qin China was the 260 BC Battle of Changping, when the Qin army massacred the entire 400,000-strong surrendered army of Zhao state (note that late Warring States period armies were de-facto conscripted national armies). Rome had the destruction of Carthago, the conquest of Gallia (Caesar himself wrote of butchering one million of whom most were civilians), the putdown of the Great Jewish Revolt and the Bar Kochba Revolt.  Etc. etc.
  
• Meanwhile, the modern European colonial empires did not usually result in mass exterminations, either. India, Indochina, Russia's eastward expansion was taken without large-scale butchering, and so was most of Africa (Kongo wasn't the model but the extreme). The Americas and Australia are special cases due to the effectiveness of diseases (though there was the Black Death, too) and the level of the techno-cultural gap.