The morally easy to kill monster is not a new idea. History is full of the concept. The dehumanization of the enemy is a long standing practice during wartime. It is easier to kill an enemy if he is not the same as ourselves. In WWII the enemy was not the Germans, it was the Nazis or "Krauts". This is done in RPG games as well. In modern-day setting games, the enemy is something evil, something alien from ourselves; drug dealers, terrorists, even Nazis and space aliens. It is "okay" to kill such enemies.
Fiction is also rife with them. The Cylons are an evil race of machines; they are not even human. Likewise, Star Wars featured a war between clones and robots. In fact, there is a reason Stormtroopers are always seen in their full helmets. It is another way to dehumanize them. The audience can cheer at their deaths without feeling guilty about it.
This brings us to the fantasy genre wherein the dehumanization of monsters is done at the outset. Most adventurers will end up fighting goblins, rust monsters and manticores. These have already been dehumanized simply because they are not human. They easily fall into the same category as killing space aliens. When humans are attacked, they are necromancers, cultists and bandits; easy to handle characterizations.
And players like this moral freedom when killing stuff. They like to kill imaginary things with no twinge on their conscious. Which is why it is a good idea for a DM to include such creatures in their setting and in their adventures. The dehumanization or humanization of an adversary should be a conscious choice on the part of a DM.
Because it is possible to humanize a monster. If a DM introduces orcs that have been “humanized” with such techniques as letting the adventurers encounter some orc babies or a good-aligned orc it removes them from being “morally easy to kill monsters”. From then on the players will need to question if they can kill such opponents indiscriminately. While this is an excellent method to get players to role-play in-character or give then another type of choice to make, it should be used sparingly. Players need their characters to kill stuff; if every kill spree is mired in moral choices it “kills” the fun of…killing.