The non-humans in question being killed are effectively the equivalent of a human bastard. When Bob kills a group of non-humans, Alice objects but Bob quickly rattles off a list of what horrible things they believed in and telling her events that happened as proof of the line of thought. Alice later sees the same species of non-humans working together with humans without conflict, Bob even talking to a few in a friendly manner.
Inverted: Human lives are casually thrown away, but killing a robot is considered murder.
Non-humans aren't killed, but are instead taken to a place where they can live together free of human oppression.
The hero has no problem crushing a robot's head... But that's because the only thing the head contains is sensory apparatus, which, while necessary in combat, isn't vital to the robot's overall survival and can easily be replaced.
...But that turns out to be a front for turning them into slave labor or Soylent Green.
As the robot fumbles around, blind, deaf, and terrified, it falls off a cliff. The hero doesn't care.
Parodied: There's an elaborate system of value attached to creatures based on how human they are, which is consulted by the characters when deciding whom to kill.
Zig Zagged: The value placed on nonhuman life varies greatly with the nonhuman. Mi-Go are killed on sight, Greys are conceded equals, Demons are considered second-class citizens, etc...
Averted: All (sentient) life is treated as equally valuable.
Enforced: Childrens' programming is not allowed to show people being killed, so there has to be something for the heroes to shoot at.
Lampshaded: "Would you still have killed him if he had been a human?"
Invoked: "Look, nobody cares about those robots. We can kill them all we want".
Exploited: Emperor Evulz keeps all the robots or undead back at the base doing menial things, and commands only humans to attack heroes, in hopes of exploiting the higher value they put on human life.
A human character refuses to kill non-humans on the grounds that they are just as good as everyone else.