Leviticus 19:18

Do not hold a grudge: One asks: "Can I borrow your Ax?" and he replies: "No"...The next day the other asks him: "Can I borrow your ax"? And he says: "I will not lend it to you just as you did not lend me yours", this is revenge. What is a grudge? One asks: "Can I borrow your Ax?" and he replies: "No"...The next day the other asks him: "Can I borrow your ax"? And he says: "Here you go, I am not like you who did not lend it to me", this is a grudge, that he guarded hatred in his heart even though he was not going to take revenge.

And you shall love your fellow as yourself: Rabbi Akiva said: "This is a fundamental principle in the Torah.

Be not revengeful, nor cherish animosity against the children of thy people; but thou shalt love thy neighbour himself, as that though there be (cause of) hatred with thee thou mayest not do (evil) to him: I am the Lord.

Do not take vengeance, and do not bear a grudge These terms are defined in the writings of our Sages [e.g., Yoma 23a].

love to your fellowman as yourself Many people have said that the lamed is superfluous, like the lamed in “they killed Avner” [II Samuel 3:30] [literally: they killed to Avner]. In my opinion, however, it contributes to the verse’s meaning. One must love the good that pertains to one’s fellowman as much as one loves the good that pertains to oneself.

I am God means: I, a single God, have created all of you.

AND THOU SHALT LOVE THY NEIGHBOR AS THYSELF. This is an expression by way of overstatement, for a human heart is not able to accept a command to love one’s neighbor as oneself. Moreover, Rabbi Akiba has already come and taught, (further, 25:36), thy life takes precedence over the life of thy brother.” “Your life takes precedence over the life of your fellow-being.” Rather, the commandment of the Torah means that one is to love one’s fellow-being in all matters, as one loves all good for oneself., 6:3). It is possible that since it does not say “and thou shalt love ‘eth rei’acha’ of one’s neighbor as much as one loves one’s own self. But instead the verse says l’rei’acha, which means “to [or ‘for’] your neighbor,” thus teaching that that which is good “for” your neighbor you should love as if it were good for youself. as thyself,” but instead it likened them in the word ‘l’rei’acha’ [which literally means “to” thy neighbor], and similarly it states with reference to a proselyte, and thou shalt love ‘lo’ (him) [but literally: “to” him] as thyself, that the meaning thereof is to equate the love of both [himself and his neighbor, or himself and the proselyte] in his mind. For sometimes a person will love his neighbor in certain matters, such as doing good to him in material wealth but not with wisdom and similar matters. But if he loves him completely, he will want his beloved friend to gain riches, properties, honor, knowledge and wisdom. However [because of human nature] he will still not want him to be his equal, for there will always be a desire in his heart that he should have more of these good things than his neighbor. Therefore Scripture commanded that this degrading jealousy should not exist in his heart, but instead a person should love to do abundance of good for his fellow-being as he does for himself, and he should place no limitations upon his love for him. It is for this reason that it is said of Jonathan’s [love for David], for he loved him as he loved his own soul, because Jonathan had removed [altogether] the attribute of jealousy from his heart, and he said [to David], and thou shalt be king over Israel,, 23:17. etc.
Our Rabbis have already explained 4:10-11. the matters of taking vengeance and guarding a grudge [which are here forbidden], that they apply to cases where there is no monetary obligation, such as, “Lend me your sickle, lend me your hatchet.” For in a case where his friend owes him money, such as because of damage that he caused him or for similar reasons, one is not obliged to let his friend go free. On the contrary, he should sue him before the court and receive payment from him, on the basis of the verse which states, as he hath done, so shall it be done to him, and he [who caused the damage] is himself obliged to pay just as he must pay back that which he borrowed or robbed; and how much more so in matters of life, [the next of kin] should take vengeance and guard the grudge against the murderer, until the blood of his brother be redeemed by a court that will render judgment according to the laws of the Torah.

לא תקם , repaying evil with evil.

ולא תטר; not even in your heart, but train yourself to resist your natural impulse to get even.

ואהבת לרעך כמוך, if he is truly your colleague, friend; however, if he is wicked you need not love him, as even G’d hates him as we know from Proverbs 8,13 יראת ה' שנאת רע, “to fear the Lord is to hate evil.” (compare Pessachim 113)

There follows a general, all inclusive rule to be observed in relations towards one’s fellow, phrased as ואהבת לרעך כמוך, telling us to apply the same yardstick to our concern for our fellow that we would want applied to ourselves if we were in his shoes in similar situations.