Here is an excerpt from the German philosopher Arthur Schopenhauer's Counsels and Maxims which ought to provide you with a good thesis for an essay on the subject of judging people.
A man bears the weight of his own body without feeling it, yet he feels that of every other which he tries to move. In the same way, he does not notice his own shortcomings and vices, but only those of others. Instead of this, everyone has in others a mirror wherein he clearly sees his own vices, faults, bad manners, and offensive traits of all kinds. But in most cases, he is like the dog who barks at his own image because he does not know that he is looking at himself, but thinks he sees another dog. Whoever finds fault with others is working at his own reformation. And so those who have the inclination and are secretly in the habit of subjecting to a searching and sharp criticism other people’s conduct in general, their commissions and omissions, are thus working at their own improvement and perfection. For they will possess enough justice or pride and vanity to avoid doing what they so often severely censure. The opposite holds good for those who are tolerant; thus hanc veniam petimusque damusque vicissim (“We beg this freedom for ourselves and likewise grant it to others,” Horace, Ars poetica, II). The Gospel moralizes prettily on the mote in the eye of one’s neighbour and on the beam in one’s own; but the nature of the eye consists in looking outwards and not at itself. Therefore to note and censure faults in others is a very suitable way of becoming conscious of our own. We need a mirror to improve ourselves.
It is not only impossible to refrain from judging others, but it would be foolish to go through life trying to do so. There are too many people in the world who can injure us in one way or another. Furthermore, it is a natural human instinct to judge everyone we encounter, if only for a brief moment, just to see whether they might be friend or foe. Jesus himself judged people. He called his contemporaries a "generation of vipers." Pretty judgmental! But Schopenhauer puts his own spin on the injunction "Judge not, that ye be not judged." He offers the excellent advice that we should use our judgments of others for self-improvement. If we dislike something in another person, it seems like a good idea to ask whether we are guilty of the same thing ourselves. As Schopenhauer says, "Therefore to note and censure faults in others is a very suitable way of becoming conscious of our own." And most of us have plenty!
There are several sentences in the above quote that could serve as thesis statements. For example:
Whoever finds fault with others is working at his own reformation.
...to note and censure faults in others is a very suitable way of becoming conscious of our own.
...everyone has in others a mirror wherein he clearly sees his own vices, faults, bad manners, and offensive traits of all kinds.
...those who have the inclination and are secretly in the habit of subjecting to a searching and sharp criticism other people’s conduct in general, their commissions and omissions, are thus working at their own improvement and perfection.
An essay based on such a thesis could consist mainly of examples of behavior we dislike in other people, along with confessions of our own comparable faults. I could think of many kinds of "vices, faults, bad manners, and offensive traits."
There are many ways you can construct a thesis on judging others. Let me give you a few examples.
First, you might want to write about the danger of judging others. If you go in this direction, you can mention the dangers of judging wrongly. How many times have we been wrong or drawn the wrong conclusion? You will have a lot of evidence with which to work.
Second, you might want to go in a more religious direction. If so, you can mention the words of the New Testament. Matthew 7 states:
“Do not judge, or you too will be judged. 2 For in the same way you judge others, you will be judged, and with the measure you use, it will be measured to you. 3 “Why do you look at the speck of sawdust in your brother’s eye and pay no attention to the plank in your own eye? 4 How can you say to your brother, ‘Let me take the speck out of your eye,’ when all the time there is a plank in your own eye? 5 You hypocrite, first take the plank out of your own eye, and then you will see clearly to remove the speck from your brother’s eye.
Third, you might want to talk about topics such as prejudice or racism. If you go in this direction, then you can talk about many of the great evils in our day and age.
Here is a sample thesis:
"While we have to make judgments in life, it is wise to hold judgments as long as possible until enough facts are gathered."