I totally agree with Sonamdhargey , that this man is a murderer . It is clear that his cold blooded execution of his victims without remorse shows his intention of handing out his version of justice. His act of vigilantism may just be so labelled so that he may carry out with passion his violent acts. What defines vigilantism is purely the concept of "an eye for an eye ", if this was condone there were to be no end to the killings.
In recent years many scientists and some religionists have used the expressions like 'humane killing', 'mercy killing', 'gentle killing', vigilante killing and even 'painless killing' to justify the ending of a life. They argue that if the victim feels no pain, if the knife is sharp, killing is justified.
Buddhism can never accept these arguments because it is not how the killing occurs that is important, but the fact that a life of one being is terminated by another. No one has any right to do that for whatever reason.