Summary:
A killer can't be born, but hatred can be born. Hatred can be a coping mechanism to combat or work with fear, which can happen nearly instantaneously after birth.
This writing is only based on observational data, there will probably be cited papers/articles/posts as I go on.
Existing conclusions:
Some psychologists believe that hatred is a learned emotion, not something that is inherent at birth. They believe this learned behavior can rear its ugly head in the form of prejudice, bigotry and hate crimes.
https://www.medicinenet.com/are_we_born_with_hate/article.htm
No. Usually if someone hates you it is because they are jealous for some reason. Not everyone is consequential enough to be hated. Also just because you don't ...
https://www.quora.com/Is-hatred-something-you-are-born-with-or-something-you-develop
Being born with hate is still a debatable question. Some believe that hate is an acquired trait because newborns have a clean slate and are conditioned according to their social and political environments
https://www.medicinenet.com/are_we_born_with_hate/article.htm
Dog World:
As we know, dogs bark a lot, and of course, it's not always a sign of aggression, but why do some dogs bark as a sign of aggression for apparently no reason, despite perhaps living in perfect conditions? Because of Fear. Discrimination.
And we know that because they bark at people they don't know until they get used to said person, so clearly barking in that initial state is a fight-flight response to potential danger which is usually going to be caused by fear, anxiety, or curiosity.
However, let's not forget that many dogs fight for apparently no reason as well. They see each other, start barking, and become aggressive. A dog can, of course, choose to be passive if they are scared or if they were taught differently.
But you see, that's when both dogs have a different response to the same trigger. One can be scared and want to fight (a person can be scared of spiders but yet still crush them with every chance they get with their increased speed and reaction time) or the other can be scared and run away (similarly, a person can be scared of spiders AND be afraid to crush them).
Now, fear can be either innate or learned with direct classical conditioning techniques. You can even turn a dog lover into a dog hater, but you see, everyone has a different response to fear.
Dogs bark to communicate. They use body language and expression, and that's really all they can do. But unlike the way humans speak, dogs have no established pattern of communication with other dogs, such as two barks = friendly, one bark = danger.
And because of the communication limitations, there's another interesting key difference between humans and dogs:
They aren't intelligent enough on a communicative level to isolate thoughts from action or feelings from action, and instead everything naturally happens at once. Basically, what this means is that if a dog hates you, you'll know they hate you. The dog won't have to bark because, once again, they are more expressive than humans with just their body language, while if a human hates you, they can be polite and you probably wouldn't know that they hate you.
You can condition a dog not to do certain things, but their body language will always be more visible, this includes if you tell your dog not to bark at someone, you'll very clearly see that they still want to bark.
From the natural reactions of fear, a dog may simply bark at another dog because they are scared, but WHY? because of discrimination. Technically, disliking someone because you don't know them is also discrimination. This includes a dog being scared of another dog simply because their fur is a different color.
discrimination, in psychology, the ability to perceive and respond to differences among stimuli. It is considered a more advanced form of learning than generalization (q.v.), the ability to perceive similarities, although animals can be trained to discriminate as well as to generalize.
the unjust or prejudicial treatment of different categories of people or things, especially on the grounds of race, age, or sex
Since humans are better at masking their emotions, at least at a surface level, it's possible for one to develop sudden hatred without knowing that there's apparent hatred. Someone may dislike dating white people because of hatred, but to them, they may just say something typical like: "just preference" and they could have white friends, but dating is a different stimulus of their discrimination that can be triggered by a multitude of things from fear, anxiety, experience, stereotyping, etc. stereotyping because of experience-> fear-> hate, which then can be replaced with "dislike" e.g. I dislike dating white people. Translation: I hate dating white people. Many can also be persuaded by social norms, e.g., saying "dislike" because apparently, "hate is a strong word" or to avoid being accused of being racist.
Racism:
You don't have to be taught to dislike a black person; you can dislike a black person naturally from other forms of discrimination triggers. It could even develop after birth. Your family is white, you're white, and all of a sudden this black figure appears.
That could actually cause fear, which can become hatred. Similarly, with a black child potentially witnessing a white person for the first time, the same type of action can happen, and I'm sure many of us have witnessed a baby just start crying all of a sudden after seeing someone (I know people that have done that, including running away from strangers that try to pick them up).
Your child could also adjust to the behavior of what they see based on these colors. For example, if a fight occurs, the white child sees the black person attack their white father, the mother is yelling and such, and now the baby can potentially register all blacks as dangerous from the momentary PTSD.
I know many people think life is just "black and white," but the reality is life is far more dynamic than that.
It's not going to always be a parent's fault because discrimination can just happen naturally based on either our brain's dopamine rewarding system or fear/indirect conditioning.
Now you're probably going to ask: "Well, how does the discrimination system work?" How will we just become scared of a different color? What causes us to become scared in the first place? "
After a baby is born, they are, of course exposed to all sorts of colors and volumes of sound, but the areas of the brain that process such information, such as visual data (thalamus, visual cortex, LGN, etc.), your nerves and brain all have thresholds of light. Based on the development of the child, different colors will cause different sensitivities or be processed differently in a more disorganized fashion (such as with an injured brain) or premature development that can affect ongoing personality changes. The baby will then naturally adjust to everything according to how the colors were processed, and if it's too sensitive, it can cause pain, crying, fear and their brain will learn to adjust and reject whatever caused a horrible reaction. This could include a white person, a black person, a green light, it doesn't matter.
During the lighting phase, other data will be processed such as size, shape, etc. with addition to volumes of sound that may also trigger pain or fear. With all of this information near instantaneously, a baby will either hate you, like you, or be neutral as they adjust to the rapid environment. Even if you never teach them what hating is, that won't stop the discrimination. For one, discrimination can be a neurological development. Hatred can also be neurological or psychological.
They will be naturally conditioned to different stimuli and it could cause fear (once again, everyone can have a different response to fear). It can either be an aggressive-like fight or a passive aggressive flight or entirely passive.
Essentially, what this all means is that once the baby is born, hatred can exist near immediately from the surface, but what actually occurs is fear or pain -> Hatred as a coping mechanism to combat or avoid it. -> When they get older, they will finally choose how they really want to handle it: kill, avoid, deal with it in a polite manner, etc.
The conclusion is that hatred doesn't have to be taught or learned (it can be learned), it can also exist naturally and that can either happen quickly or slowly with or without the knowledge of hatred. Racism can also occur naturally with or without the knowledge of racism.
Experience. Fear. Pain.
Clarification:
It is understood that "learned" in this case refers to psychological changes or this:
"An inherited trait is a characteristic that is passed from parents to their babies (offspring). A behavior is a particular way of acting.Inherited behaviors are called instincts. Learned behaviors are not inherited but learned from others."
"In general, a learned behavior is one that an organism develops as a result of experience." Learned behaviors contrast with innate behaviors, which are genetically hardwired and can be performed without any prior experience or training. Of course, some behaviors have both learned and innate elements."
So the translation from these definitions and the quotes at the beginning of this post is: "Hatred can only be learned from others or be learned through experience from others"
However, logically, that wouldn't always be the case, because neurological defects can cause immediate disapproval from pain, so hatred will always have the potential to be born. "Born" in this case does not refer to the early stages of development in your mother's uterus, but rather the initial afterbirth stages of your 1 second life, during which there will not need to be a reason from psychological experience for your hatred to develop (it won't need to be triggered by sociological events but it can occur for no reason due to neurological defects).
Not all "dislikes" are equal to "hate" however it's possible for someone to "dislike" without knowing that they actually hate.