Most killers are either psychopaths, sociopaths and narcissists, the common factors between these three is remorse. Narcissists often have a lack of it or don't feel it at all. Sociopaths don't feel it at all. Psychopaths may feel it, but their mistakes don't affect their future actions.
They all don't feel remorse or don't learn from their mistakes, this means that they do not care about consequences and moral implications. Which are some of the only reasons that everyone isn't a murderer. Majority of people do have people they want to kill, some won't do it out of fear of getting caught, others, because it is wrong. But most people deep down have a desire to kill. Some are stronger than others.
The people who are killers are just people who basically do whatever they want, they don't care about the moral implications, some, but not all also don't care wether or not they get caught. Think about it, if you knew that you wouldn't get caught, would you kill someone who deserved to die? Most likely, your answer will be yes, of course there are exceptions and not everyone would, but majority of people would, and that is how the killers feel. Either their desire to kill is so strong they feel as if they must. Or they are almost absolutely sure they won't get caught.
