The conventional way to judge people is by their behaviors, more specifically by their bad behaviors. In other words we often tend to evaluate people at their worst and not their best. This is especially evident, when we go way back in a person's past to emphasize a indiscretion or malfeasance which occurred decades ago and thereby condemn and dismiss them now.
Those of us of a biblical persuasion are probably familiar with the scripture which says that God judges on the heart and not outward appearance (I Samuel 16:7). I suspect that most take this passage to mean that God doesn't care about our physical appearance (short or tall, beautiful or ugly) but rather focuses on our inner person. However, a part of our outward appearance, which we humans love to use to judge, is a person's behaviors. To our mind good behavior defines a good person and bad behavior designates one as a bad person. Of course, behavior is a part of the outward appearance. The assumption we make is that behavior is a reflection of the heart, the inner man, the part God judges by. This assumption makes us actually feel godly when we judge and condemn one another based on behavior. We judge on outward appearance but think we are judging based on the heart, which is actually known only to God.
David is the specific example cited in I Samuel above. He was not physically impressive like Saul but God judged him worthy anyway. However, David was not particularly impressive in his behavior. Despite that fact, which is amply recorded in the biblical account, he is said to be a man after God's own heart. God obviously did not dismiss David by judging from the outside, by making his worst behaviors a measure of his worth and goodness.
In the account of the original sin and the resulting knowledge of good and evil, we again only assume that this knowledge allowed mankind to declare some men good and some men bad. There is a distinct difference between being able to discern that some behaviors are bad and the ability to declare some people as bad. It is obvious that we all are a mixture of helpful and harmful behaviors. Their is none righteous when righteousness is measured by behavior.
Most of us have loved ones to whom we are emotionally attached. We probably observe what we might deem bad behavior even in these loved ones: spouses, parents, children, etc. Despite that experience, we don't generally declare these people to be bad and irredeemable. Instead we tend to overlook and deemphasize what we consider their bad behavior and stress that which we consider good. When using the eyes of love, we see people differently and judge them by a standard which stresses their best and not their worst. This forgiveness and forbearance that we more readily extend to close family serves as an example of how God views us and how we can potentially view mankind in general, thereby embracing the spirit of Christ.