Pirkei Avot - Ethics of the Fathers

~Level 1~
Bartenura - "one who refrains from serving as a judge" - i.e. when there is a judge bigger than him (in torah). Alternatively, he tells the litigants to find a compromise between themselves.

"rids himself of enmity" - for the litigant found guilty hates the judge. He thinks in his heart that the judge did not search for his merits.

"theft" - for perhaps the judge will make a mistake and obligate the innocent to pay thus causing theft to occur through himself.

"unnecessary oaths" - perhaps he will obligate an oath to one who is not obligated. Thus, he causes one to utter an unecessary oath. Alternatively it refers to [causing] a false oath..

"over-confident in rendering legal decisions" - without proper iyun (in depth study) and without waiting (to deliberate).

~Level 1~
Rambam - "one whose heart is over-confident" - his heart is over-confident to strengthen himself to judge without fear and awe.

~Level 2~
Rabeinu Yonah - "one who refrains from serving as a judge.." - even though it is written: "you shall appoint judges and officers.." (Devarim 16:18) and "pursue righteousness (in judging).." (Devarim 16:20), thus it is a mitzvah to judge. Nevertheless, this is when there are no other qualified judges. But all the time one can refrain himself from judging, it is better for one to cast the yoke on others. For he saves himself from many doubts...

~Level 2~
Rabeinu Yosef ben Shushan - "theft" - perhaps he obligated one who is exempt or the opposite. He did not mention cases of death penalty or kidushin (betrothals) and divorces whereby the stumbling block is much greater. Perhaps the intent is that even if one is exceedingly careful in monetary cases, since they are daily occurrences, nevertheless it is impossible for him not to err.

~Level 3~
Matanat Avot - "one who refrains from judgment rids himself of enmity" - instead of waiting for his fellow to claim from him in court, he pays immediately all that it appears he owes him. What does he gain from this? "he rids himself of enmity". For as known, in every court case, the losing side hates the winning side. This is because he always continues to think that he was right and the other person profited at his expense unjustly.

But one who is sure that the money belongs to his fellow and pays it on his own - he has no hatred towards his fellow. He does not think his fellow took his money wrongfully and certainly his fellow also does not have hatred towards him. For he did not need to fight with him on this in court or anywhere else.

~Level 3~
Tiferet Yisrael - "fool" - one who is arrogant and pushes to become a judge is certainly a fool. For he increases enemies for nothing. Alternatively, he is a fool. For he does not see the punishment waiting for him from G-d, as the talmud says: "a judge should always see himself as if a sword is between his legs and gehinom is open below him" (Sanhedrin 7a). That is to say, a sword is ready to pierce his body and gehinom is ready to punish his soul. Both can kill him when he does not judge righteously. But this fool is not concerned for all this.

(Translator: note that every person is a judge on himself every day. See the section "watchfulness" in the book "Path of the Just".)