In my brief experience teaching, one thing I've learned is something that's obvious but deceptively hard to put into practice: realizing that we see only one side of our students. The corollary to this realization is that we should not judge students based on facts not in evidence.
To illustrate, when I was a TA, I had a student who came rarely to discussion section, and when he did come, he sat in the back and dozed off. He was a rather big guy and when I saw him in class, he was rather gruff and surly when I tried to talk to him. One might imagine that my impression of him was not particularly favorable.
Then one day I was at the campus student center for some errand, and ran into him. It turned out that he worked there. He approached me and we started talking, and one thing I noticed was how friendly he was. As far as I could tell, he wasn't being merely sycophantic and trying to play up to someone who had the power to pass or fail him. But he explained to me that he worked a lot and that was why he missed the sections he did and was so tired whenever he came to class.
He was quite personable, and while I'm the first to admit that I have a hard time knowing when someone is lying to me or otherwise pulling the wool over my eyes, I was willing at least to have some empathy for his situation.
I try to think of this and similar episodes when I think of my students. Knowing what I found out about him did not really help his grade: he did rally in the last third of the semester, but only enough to scrape by with a D, but it reminded me that he was not necessarily a mean person, just someone trying to get by.
The rare encounters I have with students outside the classroom almost always drive home to me the fact that my responsibility is to judge them primarily by their performance in class. As a teacher or a TA, I have only limited insight to other aspects of their personality, and I'm not paid to be their moral judge. There are times when I am invited to write recommendations, and at those moments, I am often required to opine on the students' morals, work ethic, etc. Still, it is best to keep in mind that I can judge only by what I have seen.
I also try to remember this lesson when I "bust" a student for plagiarism or cheating. Here, I'm talking about instances of obvious plagiarism and cheating, not (what is more common) the misuse of secondary sources. Supposedly, plagiarism says that the plagiarizer is someone dishonorable and who cannot be trusted. True enough as far as it goes. If a student who I caught plagiarizing or cheating asked me to write a recommendation, I would decline (instead of writing a damaging recommendation), because I would know them primarily as my student and a plagiarizing student at that.
Yet I still must remember that the act of plagiarism really tells me only a little about that student as a person. Maybe the fact that he or she plagiarized means that they cannot be trusted with even more delicate matters, on the assumption that if someone is untrustworthy when the stakes are so low, how can that person be trusted when the stakes are higher. This is reasonable enough, but there is a counter-supposition: maybe that person is untrustworthy on something so unimportant but could be trusted with something much more important. Maybe the person who is honest when the stakes are low will turn to dishonesty when the stakes are high.