That's precisely the point, I gave you a motivation that is simple, naked self-interest. The majority of humans, because they are not psychopaths, will not be willing to kill another human for such a reason. They have to be trained into killing: that their victims are subhuman, that they're doing it for the Greater Good, and yes, that the other guy would kill me if he had the chance! Even then it's not entirely effective.

The famous Milgram experiment that purportedly shows human beings in a monstrous light, found that when required to give strangers painful shocks:

at some point, every participant paused and questioned the experiment; some said they would refund the money they were paid for participating in the experiment. Throughout the experiment, subjects displayed varying degrees of tension and stress. Subjects were sweating, trembling, stuttering, biting their lips, groaning, digging their fingernails into their skin, and some were even having nervous laughing fits or seizures.

Several experiments varied the immediacy of the teacher and learner. Generally, when the victim's physical immediacy was increased, the participant's compliance decreased. ... Interestingly, some participants deceived the experimenter by pretending to continue the experiment. In the variation where the learner's physical immediacy was closest, where participants had to hold the learner's arm physically onto a shock plate, compliance decreased.

These are the natural stress reactions from harming another human being. Those stress reactions don't exist in species that are not highly social.

A stepfather who marries into a family does not typically murder all his stepchildren to make room for his own offspring like, say, a lion does.