It is impossible to define who should never try to become a judge of any kind simply because everyone judges whether it is self, others, or things. With that said, I think the best way to formulate this question is “What should people know or understand before judging?”
Self-Judgement. This type of judgment is not bad so long as one uses it as an opportunity to reflect upon what they’ve done, what they are doing, and what they are planning on doing. This evaluation should be a simple exercise to ensure that our trajectory is in alignment with the goal of accomplishing a higher spiritual purpose. History is the foundation made of lessons learned from past experiences upon which we can build a better present. One must not spend too much time ruminating over it or pondering too much over it. Life is meant to be lived and enjoyed. It is only through experiencing life that we receive the necessary feedback on whether the nature of our desires helped or hurt us.
Judging things: “This weather sucks!” It is a far too common type of judgment that people have upon nature. What they fail to realize is that, at the same time and under the same conditions, there is at least one person who genuinely likes the same weather. It is ok to judge things that are man-made, such as money, houses, cars, etc… but my advice to those seeking spiritual self-growth is to avoid making this type of judgment at all cost when it comes to Nature. In fact, not only there is nothing anyone can do about Nature, but only judge that you can change or make better. Nature only acts in accordance with the Living Laws of Nature that is nothing but an expression of the Immutable and Perfect Will of God.
Personally: Any judgment that one casts upon others is only an accurate reflection of our personal true nature. This is why people who are positive always have the tendency to finding goodness in other people while negative individuals will be quick to point out others' flaws. This truth is clearly indicated in the Bible when it says: “Why do you look at the speck in your brother’s eye, but fail to notice the beam in your own eye.” What it truly means is that the flaws that we see onto others are abundantly dominant within us. Accepting this reality is hard, however, becoming aware of this Law can spare us from accumulating bad karma. It is further said, “For with the same judgment you pronounce, you will be judged; and with the measure you use, it will be measured to you.” To avoid making a wrong judgment, it is best to learn to understand first. When you genuinely understand, you will quickly realize that there is no need to judge because you can empathize. With true empathy, comes the desire to help without judgment or expectation which is nothing short of the manifestation of Pure Love! This is why it is also said that Pure Love doesn’t judge.
Professionally: Those who want to become professional judges must understand that there are two types of Justices. Divine Justice and Human Justice. Human Justice only offers protections and right on earth. For someone who wants to become a judge, as far as the Divine Justice is concerned, they must understand that they are fully responsible for their actions including any sentence they pronounce regardless of whether it is in the line of duty, earthly legal, or earthly lawful. This is primarily due to the fact that everyone has a will that they can exercise freely (Freewill). One can choose to become a judge as a profession or else. Once the decision is made, that binds the individual to all consequences associated with their actions. In addition, it is every person’s responsibility to learn the Living Laws of God. Just as the judge will tell you in their courtroom that “Ignorance of the Law is no excuse”, therefore ignorance of God’s Laws is no excuse either.