I think we are inherently neutral, especially at birth when we're a blank slate (babies are fundamentally neutral, neither good nor evil, up until they're old enough to differentiate themselves from others). It's our life experiences and how we respond to them that make us evil or good. I would say most people are neutral, and good people are probably more common than truly evil people, and that most of the time, evil people have gone through traumatic events or periods as children and coped the best way they could, up until it became a part of their personality.
However, I would define an evil person as someone who lacks empathy, purposely destroys others or others' possession for their own gain, don't care about the consequences of their actions, feel no remorse, someone who acts only on their brutal impulses, the typical sociopathy definition if I really want to put this under an umbrella. Evil is more definable than good imo, because it's so easily recognizable. A good person, in my definition, is someone who has found a balance between helping themselves and others. They have compassion for their fellow man (and animal, though not all hold them high in their consciousness, which I understand), try to help whenever possible, but doesn't do so in a selfish way. They do it out of a genuine desire to take care of another.
Most serial killers had an awful childhood, suffered abuse, neglect (in any forms, even spoiling a child is a form of it, because no amount of toys or expensive things can replace a parent's genuine attunement and emotional mirroring), bullying, and they either didn't have the resilience or weren't taught how to cope, and the pain in them grew, and grew, and grew, up until it crystallized in them and created the monster on the inside that basically acts in a defensive way. And of course, at this point, they don't even have any self-awareness or conscience left anymore, so they don't always realize or care to think about their actions.
The best people, imo, are the ones who realize their own evil and choose to change. They are aware of their own destructive potential, but have chosen not to be this way, and instead strive to be the best version of themselves.