How can a persons good or bad be accurately measured? The majority of people who think certain people are bad people are usually because they have been exposed to the bad things they do or also because they measure how bad they are based on only the bad things they do, and vice versa with good. There's several very ethically and morally good people that would never be able to harm anyone regardless of circumstance but there's also good people (on the same scale as the previous example) that (like me) would not think twice about killing someone if it came to a life or death situation or something similar. How could a persons real good or bad be evaluated in an approach that would be decorrelated of errors, unbiased, and as accurate as possible? Would it be best to weigh their wrongdoings?the good things they do? Both? How would they be weighed to provide the most accurate result?
In a way, I agree with the answer given by AivanF.

But more can be said. One of the greatest biologist of our times, the scientist Edward O. Wilson has something to say about what humanity has been calling good and evil, regardless of any particular world view.


  "What have been thought of as moral sentiments are now taken to mean
  moral instincts (as defined by the modern behavioral sciences),
  subject to judgment according to their consequences. Such sentiments
  are thus derived from epigenetic rules -- hereditary biases in mental
  development, usually conditioned by emotion, that influence concepts
  and decisions made from them. The primary origin of moral instincts is
  the dynamic relation between cooperation and defection. The
  essential ingredient for the molding of the instincts during genetic
  evolution in any species is intelligence high enough to judge and
  manipulate the tension generated by the dynamism. That level of
  intelligence allows the building of complex mental scenarios well into
  the future. It occurs, so far as is known, only in human beings and
  perhaps their closest relatives among the higher apes."


That dynamism is, in a way, the old issue of good vs. evil:


  "We are determined by the interplay between individual and group
  selection where individual selection is responsible for much of what
  we call sin, while group selection is responsible for the greater part
  of virtue. We're all in constant conflict between self-sacrifice for
  the group on the one hand and egoism and selfishness on the other. I
  go so far as to say that all the subjects of humanities, from law to
  the creative arts are based upon this play of individual versus group
  selection."


So, all the science that studies these issues agreees that a moral development means a greater ability to take into account the wellbeing of something else than me (something like "I care about: me -> my family -> closest people -> my nation -> all human beings -> live on the planet..."). So, we are also talking about empathy.

That been said, there are people unable to take empathy into account: psychopaths. Differences in their brain have been found related to prosocial behaviour: they just don't care about other people's feelings or wellbeing (they can't, just like blinds can't see). So, we could state they are pure evil, from this point of view. But that doesn't mean that they can't do good, just that their motivation to do so is selfish. What differentiates people in terms of moral is not what they do (their wrongdoings or the good things they do), is why they do what they do. Is my main motivation selfish while I don't care if it damages you or not? That's the question.

So, can we state that a "good person" is empathic and a "bad person" is selfish? If we set aside psychopaths (and people with strong narcissistic traits, but explaining this would make this answer too long), we all behave in both ways, given the situation. And that doesn't mean we are bad people, that means we are human!

As the famous old sentence states:  "Homo sum, humani nihil a me alienum puto"
"I am a man: I deem nothing pertaining to man foreign to me."

So, in scientific terms and from this point of view, the only way to measure "good or bad" is measuring moral development. And we must take into account age, otherwise a two year old kid would be a demon!! ;) :D

As an example to do so: The Moral Development Scale for Professionals (MDSP), based on Lawrence Kohlberg’s theory of moral development.