Pirkei Avot - Ethics of the Fathers

~Level 1~
Bartenura - "do not judge alone" - even though an expert judge may judge alone (Sanhedrin 5a), but nevertheless, it is the way of Chasidut (piety) to refrain from this, even for an expert judge.

However, this is only if the litigants did not accept him on themselves to judge them. But if they accepted him on themselves, he may judge alone..

"do not say: 'accept my view'" - do not say to your peers who disagree with you: "accept my view. For I am an expert and even without your joining me I can judge alone."

"for they [the majority] are permitted [to do so] but not you" - since you joined with them (the other judges) and thus it is proper to rule according to the majority opinion.

~Level 2~
Rabeinu Yonah - "do not judge alone" - this is also a form of "refrain from judging" (previous mishna). Even though an expert judge is permitted to judge alone, but nevertheless it is the way of Chasidut (piety) to refrain from this. Rather he should take some peers to judge with him and help him. In this way he effectively refrains from judging a bit. For [at least] not everything is on his shoulders.

"for they [the majority] are permitted [to do so] but not you" - they may accept your view but if they don't, you are not permitted to force them to do so.

~Level 1~
Sforno - "for none may judge alone except One" - for G-d judges without need for testimony of witnesses, nor for claims of the litigants who deceive. Rather, He is the witness, judge, and litigant. Other judges need to hear the words of witnesses and litigants. Thus, there is sometimes error or deception.

~Level 2~
Pirkei Moshe - "none may judge alone except One" - no one can judge alone except G-d because He is One. He is pashut (divested), absolutely pashut (divested), and there are no obstructions preventing Him from understanding the absolute truth. For He is truth itself and it is inconceivable for Him to err in any way. Therefore, no one can judge a true judgment like Him...

~Level 3~
Mahari Tab - "none may judge alone except One" - as the Rambam wrote: "none can weigh [sins and merits] except the Knowing God. He [alone] knows how to measure merits against sins" (Teshuva 3:2).

And even according to the opinion that the angels (spiritual beings) know the thoughts [of men], nevertheless this is only the thought itself. But regarding how true the thought is, this is already more inner than the thought itself. For even the man himself who is the thinker of that thought - even he does not understand on himself how much his thought stems from a point of truth. This is a deeper level. Only the Creator of all worlds knows this..

~Level 3~
Matanat Avot - everyone explains this mishna to be referring to judges, but it seems to me the mishna is also referring to each and every person who sees something bad in his fellow. Then he begins to judge his fellow negatively and thinks the fellow is a wicked person and a sinner. "How could he commit such severe sins, etc. etc."

The Tanna (sage) comes and says to this person: "my dear friend, you saw this act alone. Thus who says you are right in your perspective and outlook on your fellow? How can you be so sure that everything you see and understand is truly correct and accurate? Perhaps there are other details you do not understand in the person or his deed. Perhaps since you are missing the full picture, your entire perspective is off and if you could know everything, you would have judged your fellow more to the side of merit? Perhaps you would even think that he is completely justified?

Therefore, please "do not judge alone". Stop judging your fellow according to all that you yourself saw. For you do not know the whole picture.

"for none may judge alone except One" - only G-d who sees and knows everything absolutely, who sees all the hidden considerations behind every human deed - He alone can decide whether this act is considered good or evil and how much right and wrong it contains. But you are a simple man whose knowledge and vision is lacking and limited. Thus do not be quick to render judgment on your fellow based on something you saw in him..