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.
I think answers to your questions are fully based on your moral principles and your world view. Because there is no obvious common principles. Christians believe in objective rightness of God's rules. For atheists that is good doing, what is good for person and society. Some wild men can think that eating each other is a good thing for many generations.
In addition, have a look on stages of moral development, you might be interested in that. It is about how person's understanding of moral principles changes through time, how a person learns more general concepts.
Before we can even begin to discuss how to measure a person's relative good and bad degrees, we must define good and bad. These terms are far too vague on the surface. Even if we agreed on more specific terms such as moral and immoral, there would still be a fair amount of disagreement on their meanings. For example, moral could be grounded in religion; emotion; group survival; species survival; planetary biological continuity; perpetuity of culture, information, and consciousness; or something else entirely. Similarly, when we speak of good, we should ask ourselves, "Good for what"? Is it good for Me? You? This or that nation? Advancing technology? Destroying your enemy? Equality? Non-human animals and nature? Having fun?
Often times we hear of good being that which "chooses the group over the individual", but even in that definition I doubt we can find agreement. For example, any individual who fails to be productive would do better for the group by taking his or her own life. Would everyone agree on this type of action being the good of which we speak? Taking it further, some individuals work harder than others while consuming less resources. Would it thus be good for all but the most productive individuals to cease to exist? That would be, after all, good for the group as a whole since it would provide selective pressure toward maximum productivity and efficiency.
Another type of good is happiness. If we agreed that happiness is that which makes us feel good, then we find ourselves in a hedonistic pursuit. Virtual reality and carefully-crafted drugs could probably bring about a very large amount of this "happiness" while consuming the least natural resources per-capita. In this situation, those individuals who develop VR and drugs would be the most good individuals since they bring us further toward happiness.
Once we have defined very carefully what we mean by good and bad, we could then measure an individual against those criteria. Without formal, solid definitions we can expect the debate to be inefficient and inconclusive. It would be unrealistic to make a measuring tool for an undefined property. In that case, we might as well assign individuals random values.
To answer your main question directly: First define good and bad in a concrete way. Then develop ways to measure for each person his or her relative contributions to these definitions of good and bad. Finally, subtract the measurement of bad from that of good for a given person to obtain a general level of net goodness.