Serial killers without doubt are all mentally unwell, whether as a result of some event or trauma in their own lives, or through chemical imbalances in their brains they were predetermined to have a susceptibility for from birth, they cannot think and process emotions like the rest of us can in everyday situations.
The majority of serial killers appear to have begun life in abusive or broken homes, with little or no parental influence and no positive social interaction with family members. This leads them to develop abnormal ideas of what is normal for people to do, IE sexually abusing other people or being very violent towards others, and in repeating the behaviors they experienced and lived with, they will one day take their behaviors to the next level, which in this case is killing people.
This stops them from developing a social set of normal behaviors which helps us to judge how we should react in certain situations, and how we would interact with others at a social level. Because of this they are often socially backward, or the other extreme, seem socially normal, but secretly think that they are somehow above and beyond other people, and that they are lowering themselves by interacting with normal people, meaning they tend to avoid it if they can, but when they do they appear as normal as any other person.
Many serial killers are described as sociopaths, people who cannot feel empathy, or feel sadness or guilt in a normal way like we can, which allows them to kill people with indifference and without suffering any negative emotions as a consequence. When a normal person kills another, they usually feel guilt and empathize with their victim, as well as knowing that they did something wrong.
In short they can put themselves in the victims position in their mind, and can imagine the victim missing out on the rest of their life, and knows the full impact of what they have done.
Sociopaths however don't see any of this, and because of which can kill indiscriminately at will without feeling any guilt about it at all, even if their victims were totally innocent of harming them or were randomly chosen.